For yellowed glass keys on typewriters, there's usually a key top covered by a piece of paper with the key glyph on it which is sandwiched in with a small piece of glass and a metal ring that holds it down with several metal tabs underneath the key to hold it all in place. There are custom keyring pliers for quickly removing and replacing the papers which needs care not to crack the glass. Otherwise you can manually bend the metal tabs on all your key rings to remove them and replace the papers. (This is generally a LOT of work either way.) See: https://www.reddit.com/r/typewriters/comments/1csni4d/neat_find_on_clients_kmg/

I prefer the yellowed patina of the older key papers, so I tend to leave them versus spending the time and effort to replace them.