mobile

I want to give the awesome Android 5.1 experience to every Galaxy S2 user. I am here to help, so if you need help just post a new replay.
I will upload all I can on my own Google Drive, so when the Link is down someday send me a message.
This thread is ONLY for the Galaxy S2 GT-i9100 modell! Check it by going into settings -> about phone -> modell!
I am here to help, so if you have questions or problems post a reply here and I will give my best to help you. :)

Your warranty is now void.
I am not responsible for bricked devices, dead SD cards,
thermonuclear war, or you getting fired because the alarm app failed. Please
do some research about all the things I tell you
before doing it! YOU are choosing to make these modifications, and if
you point the finger at me for messing up your device, I will laugh at you. Hard. A lot.

Pre-requisits:
A1. Download Odin 3.07: HERE
A2. Download newest CWM Recovery:
A3. Download Phone Drivers [most not needed so only download when your phone doesn' get detected from your PC]: COMING SOON!
B1. [Optional - only if you need root on your Stock Rom] Download CF-Autoroot:
B2. [Optional - only if you don't want to have a so called Triangle which says that your device doesnt have OFFICIAL software anymore, check more!] Triangle Away: HERE
C1. Download your Android 5.1 Rom, here you can find some, but there will be some more:
- CyanogenMod Unofficial [Most known Rom]:
- Resurrection Remix Official [Most features of all these Roms, very smooth]:
- OmniRom Unofficial [Simple but clever :)]:
C2. [Optional but really recommended!] Download Google Apps: HERE [TAKE ONE FOR ANDROID 5.1!]
C3. [Optional!!!] Download Apollo Custom Kernel, this will also bring TWRP Recovery to your phone (what I do prefer much than CWM): HERE and don't forget to install the Config App on your PHONE:

A. So first let us start with giving you a new Recovery. I decided to show you because this is the most known one and there is no TWRP for it.
- Go to your PC and open Odin307.zip, now extract it somewhere and open Odin3 v.3.07.exe
- Take your S2 and bring it in Recovery Mode by shutting it down and then press Power+Home+Volume Down
- Now take your i9100 Galaxy S2 and connect it to PC with a USB Cable [it is recommended to use the cable which was delivered with the device]. Odin should now says something at ID:COM and under message should stand: "Added!".
- Check in Odin if ONLY THE FOLLOWING is crossed under Option: - Auto Reboot; F. Reset Time! NOTING MORE
- Now click in Odin on PDA and search your downloaded file called something like: "CWM-KitKatCompatible-i9100.tar"
- Let Odin check the md5 and after this is finished press "Start". Give your phone some time (up to 10 minutes) and wait until there is "Passed" in Odin.
- Your device has now lost garantee (only when you don't use Triangle Away) and you have a custom recovery

B. [OPTIONAL!] When you want to use Triangle Away or need root a last time on your Stock Firmware you have to root it first.
- Open AutoRoot S2 i9100.zip, now extract it somewhere and open Odin3 v.3.07.exe from inside the folder
- Take your S2 and bring it in Recovery Mode by shutting it down and then press Power+Home+Volume Down
- Now take your i9100 Galaxy S2 and connect it to PC with a USB Cable [it is recommended to use the cable which was delivered with the device]. Odin should now says something at ID:COM and under message should stand: "Added!".
- Now click in Odin on PDA and search your downloaded file called something like: "CF-Auto-Root-XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"
- Let Odin check the md5 and after this is finished press "Start". Give your phone some time (up to 10 minutes) and wait until there is "Passed" in Odin
If you now want to use Triangle Away read more at
@Chainfire
's Official XDA Thread, linked at pre requisits!

C. Now we are at the Step to get your new Android Vanilla 5.1 Software. Go to the threads upside and take the Rom you most like, if you need help to decide I can help you :)
I would recommend you to use Resurrection Rom because for my S2 this was the smoothes one.
- Copy your downloaded Rom.zip and Gapps.zip to your micro SD Card from your S2
- Shut down your phone, then boot into Recovery by Holding Power+Home+Volume Up, release Power as soon as you see the "Samsung Galaxy S2 i9100" Logo, otherwise it will maybe reboot again.
- [Optional but recommended] You will see the TWRP Logo, then go to Backup and Restore and do a backup!
- Go to Factory Reset and do a factory reset, then go to wipe cache and wipe cache, then go to advanced and wipe Dalvik, then go to mounts and storage and wipe System, Data, Cache, boot and Internal Storage (default)
- Now go to install and choose you directoy where your downloaded zip's are, and choose your Rom, then press install again and choose your Gapps.zip and wait until it has finished
- [Optional] After finished installation go to Advanced and Wipe Dalvik again. Then reboot your phone.
Finished! You have now the newest Android 5.1 on your S2!
OPTINAL you can now install a custom Kernel but this is really OPTIONAL! First read something about custom kernels before doing so!
Download the zip and flash in Recovery, finished, then install the Config app on your phone. CHECK FIRST IF THE KERNEL WORKS FOR YOUR ROM! Apollo works for CyanogenMod, Resurrection Remix, the rest I don't know.

Please press THANKS if you find this Thread usefull!

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Also Check:
- How to Root Galaxy S6 SM-G920F: HERE
- How to Root and install Android 5.1 on i9300 Galaxy S3: HERE