how can i edit itunes.xml?

Some of my songs have an exclamation mark to show they can't be found but they show up in my playlists. I would like to delete them but I can't because they can't be found!

Maybe I need to edit them out of itunes.xml if this registers everything that has ever been loaded into iTunes.

I would also like to know how these songs go missing. I can't find them in Time Machine.

I have an iMac from 2008 and a more modern iPad. I stopped updating the iMac after 10.11 El Capitan but the iPad is up to date