How to flash stock ROM on Galaxy Young
This may  wipe all user data!!!

So make a backup just in case!!!!

You need to flash your appropriate firmware package of your device, download it here:

http://forum.xda-developers.com/showthread.php?t=2568853

Download Odin (google it)
Open Odin (duh)

Reboot your phone to download mode.(google it if you don't know how to)

Connect your phone to the computer

Put the firmware package in PDA/AP

Klik start to begin and your phone will reboot with "new system files"

Hope my answer helped.