Assuming you have version 127.5 (it shows the version on the LCD when you turn it on):
Hold 1+2 until the menu appears. Press 1 until it reads CompMode then press 2. Press 1 until it reads Semi then press 2 to return to the normal screen.
If the competition lock is on (if there is a L on the screen) then you will need to disable that before you can change fire mode. That requires opening the grip and using a paper clip. I can show you how to sort it out on Sunday if you have trouble.