My suggestion is to proceed with this kind of tweaking only if strictly necessary. The pitchwheel is mapped by a nonlinear function to two semitones. By extending this range, you may be disappointed by the results.
Anyway, this is how to change pitch bend range.
Load the instrument. Select: Edit all groups -> source -> modulation.
The cursor is now set to 2.0 % (two semitones). Move it to the desired value, and you're done. Don't forget to save under a different name.