Go into the Key assign area on the first page. It will have listed...
Trim Func step
Trim 1 trm s 5
trim 2 trm t 5
trim 3 3ch (this area is where you need to make changes)

Go to the trm3 step area it will have a number. One you have highlighted it use the incline and decline keys to change that number. For the emaxx 2 speed you will want it at 200 this will make it so when you hit the trm 3 switch the servo will flip to the opposite side or where it previously was making it shift gears.
I hope this helps.