How can I remove old devices from my iTunes-Mediathek?

Question

How can I remove old devices from my iTunes-Mediathek?

Answer 1

That's a very general question, so here are two documents from Apple for using automatic syncing and manually managing content.
Sync your iPhone, iPad and iPod with iTunes using USB - Apple Support
Manage content manually on your iPhone, iPad, and iPod - Apple Support
If your question is actually about removing content (you know how to add content), you are probably using the manual method to add content, and don't know how to manually remove content.  For iTunes 12, on the horizontal bar that goes across the iTunes window (near the top), click the Music button near the left end.  Near the middle of same bar, click Playlists.  The sidebar appears along left side of window and Devices, your iPod is listed.  Click the small triangle to the left of the iPod's name, to drop down its content list (indented below iPod).  Click Music there.  To the right, the list of songs on the iPod is shown.  Select the song you want to remove and press Delete on the keyboard.  You can do this with one or more songs selected.
You can also use the iPod's "management" screen.  Select the iPod in iTunes, so that you see its Summary settings screen.  In the sidebar, under On My Device, click Music.  This is the same as on the "content list" described above.