@Bart
Yr post is the "nearest" answer to my general question how to export/edit/re-import gpx file(s). I was trying this with limited success. Here are my "open" points .. and I hope you (or someone else) can help with a general attempt.
I'm using
-- OSMAND+ 3.1.5 on Android 8.0.0
-- Linux/Mint 19 (Tara)
To download the .gpx files (like favorites.gpx) there is no problem. And with the hard way to edit the .pgx file using an editor (atom) is OK also.
To upload that edited file back to OSMAND+ installation seems to be a problem. If the name is different like 'favorites1.gpx' it's shown in the Android directory, also opening the file in external program like 'GPX Viewer' (vers. 1.29.8) is OK, the favorites are shown on a google maps.
The problem is to "replace" or overwrite the normal favorites.gpx file.
-- to upload it with the Android/Linux cable link, also no error msg, the file isn't changed on the Android.
-- renaming with using the link from Linux or also on Android with a file manager fails.
(with Android: SD-Card --> Eigene Dateien -> Android -> data -> net.osmand.plus -> files -> favorites.gpx)
There is reported OSMAND holds the user/app data also at some other places, but those infos seems to be (very) old, so I'm not sure what are the true right locations. Probably the location is the primary problem? Any help for that?
Last not least, what is the best way to edit the favorite and track gpx data. What are the "experts" using?