Samsung may have abandoned android development for the Samsung Galaxy Grand Prime but folks at xda have been supporting this entry level android phone quite nicely. One of the developers recently released a Pac-Man ROM that brings android 5.1.1 to the Grand Prime.
* Download the ROM and GApps
* Reboot to recovery
* Wipe data/factory reset
* Flash the ROM and then GApps
* Reboot your phone
Installation instructions:
* Reboot to recovery
* Flash the ROM
* Wipe both dalvik cache and cache
* Reboot your phone
It is obvious but flashing this will void your warranty but since Galaxy Grand Prime is already a pretty old, there is nothing to worry about.