How to change your psp version from version 6 to 5?

Someone had lent me his PSP game (It was an UMD disc). Before entering the game, I had to change my version from 5 to 6 (I got a notice). After finishing the game, I decided to play games in my memory card but I got a notice that there were no games although the games were in the card. I tried many ways to get them back but I could not. I need to know a simple way how to change my psp version from 6 to 5.