How do I set video recording rate to 60 fps or 120 fps and change between the two?
the only option using the iphone's camera is by switching between Slow-Mo and Video modes. You can then speed up a Slow-Mo video to normal speed in the photos app pretty easily. You can't convert normal videos to Slow-Mo.

There are probably a ton of apps that offer a bit more manual configuration than that. But I don't know of them.