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.

Keyring pliers are used to remove the metal rings off of both circular and tombstone glass typewriter keys so that the legends can be replaced or even covered over with black paper circles for teaching or learning typing. They take some practice and skill to use, but speed up the replacement of legends significantly.

Wiping dirty plastic typewriter keys with a soft cloth and then waxing/buffing them can bring them back to life.

Smith Corona Vintage Typewriter Straighten Chrome Ring Keytops Twisted Crooked Letters Repair by [[Phoenix Typewriter]]

Royal Quiet De Luxe Typewriter Sticky Keys Segment Flush Clean by [[Phoenix Typewriter]]

https://www.reddit.com/r/typewriters/comments/2rru4x/where_can_i_buy_the_key_to_the_case_of_a_smith/