But to install new rom you can find them at Sammobile.com, flash it with Odin .
Then you can install a custom recovery with Odin and root it (after root delete all Knox app which make the rom instable).
The stock rom don't wipe your datas.
To install custom rom use a custom recovery (TWRP or CWM).
Very often custom rom need to wipe your datas to have a clean install !