I exported to wave for windows media, I did not save file as aup, can i re-import or send file back to Audacity to edit labels/tracks etc or do I need to re record cassette tape and start over?
If you exported the work as a WAV (Microsoft), you can just import it and keep going. It will be only stereo or mono, though. If you Saved a Project, all the individual tracks in the mix would come back—however many you had.
Projects do not save UNDO.