There are two buttons on the 3Doodler PRO. The circular button is for extrusion, and the triangular button is for reversing. A single click of either button will initiate its respective function. Another click of either button will stop extrusion or reversal.
If the material is long enough you can reverse it out by pressing the reverse button once, then pulling the strand out once fully reversed. If the material is not long enough to do this, you can remove the nozzle (always with the pen on and heated) and insert the thin unblocking tool through the nozzle end of the device. This will push the filament out through the rear.
When clearing the pen, keep the pen at the temperature of the last material used, allowing all of the previous filament to extrude fro the nozzle, before switching to the new temperature.