Mike - if you want to change a variable channel to fixed, this is the procedure from you remote:
Menu - System - DCS Setup - Variable 1 - hit soft key FXD to set the channel to fixed.
However, you don't really need to do this just to test to see if a fixed channel is not working. You can run command from a variable channel w/o permanently changing it to fixed. Just swap the input and output from the problem fixed channel to a variable channel and make sure you bring the voltage up fast enough so the engine does not think it's being operated conventionally.