Originally Posted by ApolloZero
You can flash the latest Firmware with Odin.
It's a very simple Tool and all your data+apps will be stored.
Some stock firmware you found in this thread
Hi, if you are updating to a new major Android release (i.e. from 4.1 to 4.3) it is strongly recommended to do a full wipe.
In order to keep your data, it is best to run backups.
Some users experienced problem with their EFS when changing firmwares, so an EFS backup is a must.
Check out this thread for details:
> Samsung Galaxy S III I9300, I9305 > Galaxy S III General > [REF] IMEI / EFS - Know this stuff (i9300) - including "no network"
Titanium Backup is an excellent tool to backup your apps and the app data (root is required).
It is also recommended to copy the content of your internal SD-Card on a PC (and also the content of your external SD card, if you have one).
If you install a custom recovery, you can make a backup of your whole system including apps (i.e. CWM, PhilZ, TWRP).