I'm fairly certain you can't change the framerate mode at runtime. You can set the fixed fps rate, when you selected "Fixed" as mode in the runtime properties. That's about it.
[quote="namre":1iban8rs]The reason why I am asking is because I want to have an option where users can set whether they want V-Synced or not.[/quote:1iban8rs]
Fixed mode reduces the display quality significantly. Also if you "code" your game properly, it should run correctly in Vsync mode on all computers. If you had improper events, mostly involving Timedelta errors, the game would appear different on different computers depending on the refresh rate. Did you consider this already?