After a very productive iD editing session, I got this error:
Errors occurred while trying to save Precondition failed: Way -63 requires the nodes with id in 11365414266,11365414267,11365414268,11365430969,11365430970,11365430971,11365430972,11365430973,11365430974,11365430975,11365430976,11365430995,11367299006,11367299007, which either do not exist, or are not visible.
I was able to download a file named changes.osc, but editing it looks like a daunting task… and even if I can successfully edit it, I don’t know how to re-upload it to the iD editor.
Any advice as to how I can avoid losing the edits I made?
Your edits are definitely not lost and can be added to the map.
Acording to this post, which you may want to read, iD does not support the upload of.osc files. Other editors (like JOSM or Vespucci) do though. If using one of those seems daunting to you, others could upload your changeset for you via JOSM.
the data in its current form does only help as reference for the routes without stop and these routes should not be added as ways but need to be changed to relations using the underlying highways as members, see Tag:route=bus - OpenStreetMap Wiki.