I generally (always) set the extruder temperature in my slicing software. I use Simplify3D, but every slicer out there has a temperature setting.
If you’re talking about changing the extruder temps for the purposes of preheating / loading filament - there’s no need. The ABS setting will work fine for getting your PETG loaded, then just indicate the temps you want to print at in your slicer & generate your .gcode. Should be good to go!