It took me a while to figure this out so I thought I’d post in case it helps someone later. I installed the V3 dual extruder months ago and I had to switch back to the single tonight due to a broken nozzle on my dual extruder. It took me way too long to remember I put the toggle extension on the printer.
1 - Install the single print head
2 - Flash the Taz 6 with the single head firmware
3 - Remove the Z-axis toggle extension