The parts bin is read only so parts editor can’t edit the part there. You need to drag the part in to the sketch then select the part in the sketch (which is writable) and parts editor should be active. I will note that modifying parts is difficult and if you need only a small number of parts, probably not worth learning. The first thing to do is a google search of the form “fritzing part part_name” (where part name is the part you want) which should find any available parts. Note not all parts on the net work correctly so check that whatever you find works correctly. If you want to make parts these two tutorials apply to current Fritzing versions
I lately learned there aren’t links to the videos in Old_Grey’s tutorial so you need to do a google search for the title and then they come up on YouTube.
Old_Grey’s tutorial covers parts editor, I rarely use it and thus didn’t cover it. Good luck (unfortunately you will probably need it !)
!)