How can I increase FPS on my iPhone XR?

The refresh rate of your iPhone is something that is generally not under your control.

Some games might give you quality settings, and choosing lower quality options might allow a game to refresh the screen at a faster rate. However, the presence of such controls would be up to the app developer – and you would need to look at the settings for the particular games you use to find out if any such controls were available.

You could try completely closing out all apps other than the game that you are playing, so that apps that have open windows in the background are not using memory, but there's no guarantee that this will have any effect on refresh rate.

What do you mean by “fps”?

If your phone is lagging, what are you doing on the phone when its lagging?

Are you trying to shoot movies? Frames Per Second or is it your screen refresh rate or are you playing games and your game is slow?