See the manual PDF page 56:
http://manuals.hobbico.com/fut/3gr-2_4ghz-manual.pdf
There's a particular part of the mechanism to push and it clicks into place. I used to run mine 5:5, the only trouble is if you pull hard on the brake you can accidentally click it back to 7:3, which messes up your trim and you can't sort it out without taking the back off again. So I ended up getting used to 7:3 to avoid this happening. It may be how yours changed.