If your frame limiter is displaying 144hz in game you probably have your frame limiter set to unlimited. Keep in mind, this is 144
hz not FPS. It is just displaying your monitor's refresh rate.
The value directly below 144hz is your frame time. If that is at ~7ms, that means you are being capped to 144hz, which might mean you have vsync enabled or something. If it is lower than that, you are not being capped to anything and as long as you aren't getting performance issues you shouldn't need to worry about anything.
If you want to set a custom frame limit anyway, you will need to close the game and change both of these lines in the "osu!.<username>.cfg" file in your osu! folder, otherwise it's not going to work properly:
CustomFrameLimit = 1000
FrameSync = Custom