[Galaxy S3 Neo Android Development] Marstradamus:
How old your phone is doesn't matter, which phone you have is more important (Gt-i9300i/ Dual Sim Model or GT-i9301i/Single Sim Model) this you must figure out and load the needed Files,

the custom recovery (I suggest Rox' TWRP) and then of course a rom (those from Rox are made for both variations of our phones like his recovery) and maybe a GAPPs Pack.

Plus you need Odin and in your case with the 4.3 firmware a stockrom with android 4.4 to flash the recovery.

To figure out how to flash a firmware via Odin I recommend you this threads:

http://xdaforums.com/galaxy-s3/help/guide-odin-flash-guide-t1671969

http://forums.androidcentral.com/am...k-rom-via-odin-new-interface.html#post4265806

EDIT:

Found this thread for upgrading from 4.3 to 4.4:

http://xdaforums.com/galaxy-s3-neo/help/flash-s3-neo-i9300i-android-4-3-to-4-4-t3134643

Your Stock Rom with Android 4.4 you can grab from here:

http://www.mrcrab.net/stockrom/gt-i9301i.html?PageSpeed=noscript

(Search for a Version with your Location/near your Location - I dont know if its Important or not but I think its best to use a correct location because the baseband)

The Recovery you can get here:

http://xdaforums.com/galaxy-s3-neo/...-s3-neo-gt-t2906840/post56085899#post56085899

(choose the 2.8.5.0 version from second post, its newer)

Then upgrade your 4.3 Android with Odin to Android 4.4 then you are ready for your custom recovery, but first you should copy your preferred custom rom (CM12.1 for example) an GAPPs Package onto your SD Card.

After Odin has finished his godlike job you will be ready to boot up into your recovery, just hold volume up, home and then hit the power button.

(With my device its enough to just hold volume up and press then power, but rox said press all three keys, so you should do it :P)

Then your phone will boot into recovery and from there you first do a wipe.

To do this click on Wipe, then on Advanced Wipe and select DalvikCache, Cache, System and Data.

Then just swipe the slider to wipe the selected partitions. Then go back to home and click on install, select your SD Card under Storage and search your Rom (which you put there as i wrote before)

then click "add more zip" and search again but for the GAPPs package. After this you swipe the slider to begin the Process.After it finished you can reboot to system and enjoy your selected Rom.

If you choose CM12/CM12.1 then you can activate your root acces directly under the Settings. To do this you just need to open Settings, scroll down to "about phone" and click a few times on the build number.

This will turn on the developer options, go back to setting and open them, there you will find your option "root access" there you can select "Apps" and "ADB and Apps" - what you activate depends on what you need, for me "Apps" is enough because I dont use the the ADB.

I think I said everything Important and hope I forgot nothing, maybe someone else can confirm my little tutorial? Iam also in a leraning process.

Thats all, nothing complicated or something hard to do I think. If you got more questions please ask again if this helps you, please hit the thanks button ;)

So flash on buddy.

---------- Post added at 08:04 PM ---------- Previous post was at 08:00 PM ----------

Like Rox wrote in his post, you must flash this Zip from the link via recovery:

http://xdaforums.com/showpost.php?p=57818464&postcount=267

If I understand it right, there are two Versions of this Phone with two different Camera Sensors in it.