1.) Select the songs you want to change.
2.) Right click -> Tools -> Queue songs for tag edit.
3.) Switch to the "Tag Editor" window.
4.) Change the tags according to your wishes. Note: The changes are not executed immediately (only queued) until next step.
5.) Click "Save"
Notes to Item 1.)
You can not add songs in the "Tag Editor" window. You must therefore use the other "browser windows" like "Search", "Play Queue", "Database" ...
Notes to Item 2.)
If "Queue songs for tag edit" under "Tools" is missing, you must specify the correct path to the music files (Music -> Preferences -> Connenction -> Music Directory).