MPD is updated by sending the update command. The update command is supported by most, if not all, clients, which all have their various places they update at, alternatively if you have issues getting MPD to start, run mpd --create-db as root AFTER you've edited the /etc/mpd.conf file to correctly reflect where your music-files are located. You will then see mpd adding all your files to its database.
- Note: "mpd --create-db" is now deprecated.
Note that MPD will only rescan a file already in the database if its modification time has changed. Some ID3 taggers will restore the old modification time when changing ID3 tags. This will mean that the MP3 will not be rescanned and the new tags will not be used. In easytag 1.99.4 and later you can disable this feature by unchecking Preserve modification time of the file under File Settings in the preferences dialog (Settings->Preferences - screenshot).
More recent versions of MPD have removed the --create-db option. Instead, issue the update command from mpc (or other suitable client):