In case you want to change the reel, the first step is to bring the extruder to temperature (at least 180 °), after that you have to rotate the gear wheel by hand to extrude the filament. After extruding the filament (5-6 cm), you can open the tab and turn the wheel in reverse to withdraw the thread.

Done this, remove the old coil from its holder and mount the new one in its place.

You can reinsert the thread, making sure it is properly pulled by the sprocket.

The certainty of having carried out the reel change correctly, you will have it when you try to extrude the new material.