Very easy.
- MENU
- SYSTEM
- DCS SETUP
- Scroll to VARIABLE 1
- PRESS FXD
- Scroll to VARIABLE 2
- PRESS FXD
Note that if they're configured as fixed channels, you can temporarily drop into variable mode by just selecting the track using the TR key and varying the voltage. They'll immediately be back in variable mode until the next power cycle for input power to that channel.