How to update an mp3 tag?

Edit that field, click elsewhere. Done.

Nope, everytime I go back to the track the changes have not been saved

Are your tracks synced via iTunes?

No, just imported from the tracks on the hard drive.

Don't know what to tell you, I can edit tags, close the software, re-open and everything is there.

Are the tracks MP3 or another format?

My changes are saved without pressing the enter/return key...

You have to execute the changes by pressing Enter/Return when editing ID3 in rekordbox. Clicking out of a field will save a field only if the field is blank. Clicking out of a field will not save a field if it is being updated. You have to press Enter/Return to execute the change. I think this needs to be addressed in the next update of RB.
I don't think RB will allow to edit ID3 if you are track editing directly from the iTunes collection list as this has always been case.

Yep just discovered changes are saved by hitting enter

^ actually, I take that back.

If you are syncing from iTunes, you can try to make edits from your collection (not from the iTunes xml list - RB won't allow this anyways) and it will appear to be written. However, if you quit and reopen, it does not update to what you changed. If your collection is RB managed, I think it's completely fine.

I don't know if this is normal behavior.

So I discovered with my comment above that if I click on the refresh button in the Tag Info window, my changes would appear. If I quit rekordbox and reopen, I have to do the same again..

It seems as though the tag edits don't actually save to ID3.... It's like as if it's stored somewhere else and never gets shown unless you force a rescan of tags

Some tags are not stored to the file itself but rather into the rekordbox database. This partly depends on the kind of file you're using as some filetypes don't support additional metadata.

I guess this would all depend on how RB is taking the meta data from iTunes, which means how iTunes show the meta data. So if you write ID3 from RBDJ and do not "refresh" the ID3 tag in iTunes, then the change won't show correctly in iTunes, which means if you don't see your ID3 update unless you hit refresh in RB, then iTunes needs to rescan the tags.