Hi all,
How does one undo an upload from JOSM?

History: -
Started JOSM.
Said new version available. Downloaded and upgraded.
Loaded track, downloaded relevant data from OSM.
Edited.
Uploaded.
System seemed to hang. (Uploading for more than 15 minutes.)
Cancelled upload and hit upload again. Same result.
Exited.
In browser, saw changeset as being editing.
Come back a few hours later. Now shows it as a proper changeset. (#8637628)
The map is a mess.
NEED TO UNDO. HOW?

All help appreciated.

Regards
Sutripta