How do I limit the player fps or display hz to 60?

I am trying to limit the fps because my game plays a lot better at 60 fps but if someone play it at a higher fps like 100 fps the game plays to fast and things break a lot of code breaks i was testing the game when i had a friend try out the game and he had 100 fps u thought it didn't matter but when he was playing it broke the game why did it break the game how did it break the game how does having more fps break the game he got out of the map he broke his weapons he broke the enemy ai as well