The best way is:
1) to loosen the screws that hold the extruder motor in place 2) then to heat up the extruder so that you can pull out the screw along with the motor 3) then you quickly remove the plastic from the screw with the help of a “pliers” like tool, as long it stays hot, and the plastic is soft (If needed the screw can be inserted back into the hot extruder barrel, so that the plastic can become soft again) 4) After the screw is cleaned, you take a 12mm drill bit and gently drill out the remaining molten/very soft plastic out of the extruder barrel using your hands
That’s basically it.
Of course, you can also just purge for example PETG out with PLA. (Remember the actual melt zone is only 20mm long, not that long like in a big injection molding machine, which makes this possible)
I do this very often when I want to changes the currently “loaded” plastic with one that is very similar in its melt point. However, when the plastic you want to change has a significantly higher or lower melt point, then the “manual cleaning process” I mentioned above is inevitable
I will make a video about this “manual cleaning process” in the future to make things easier for you