I think that option is for editing the OSM “background” map for the location you have right clicked on.
How you edit a uMap depends on whether you created it while logged in or not.
If you were logged into an account you need to log into the same account again and visit the map. An edit button will appear int he top right of the screen:

If you weren’t logged in you will need to find the “edit URL” which is different to the normal URL for viewing the map. Visiting the editing URL will cause the same edit button to appear.
The edit URL appears to take the form: http://umap.openstreetmap.fr/en/map/anonymous-edit/{map-number}:{some unique gibberish}. This might be visible in your browser history somewhere?