After testing it on the reference Underwood 5 typewriter, and seeing the difference a proper bright-red key-top makes, also wanted to replace the faded paper on the color selector key of the 'good' Underwood.
This machine however, had the nickel key-ring very firmly in-place. The word 'limpet' did spring to mind.
Knowing that in case of serious mishap there'd be the reference-machine to donate a replacement, decided to try to force the keyring off. After carefully bending the tabs open, the red key was placed with the glass-side supported on a stack of cardboard disks. Gentle taps on a sharp screwdriver held on the edge of the nickel ring then got it loose, without any damage to the glass. Then going round, tapping the ring to make it move off 'straight', the keyring came off with only one small nick on the lower edge where the screwdriver pushed it. Barely visible, especially when mounted to the back and out of sight :)
With the ring off, the glass disk comes out for cleaning and a new paper insert can be cut out. In this case two disks of red paper were stacked over the original, faded paper (that had fused with the metal cup). The side of the cup was cleaned from dirt and corrosion, and then the keyring pushed back firmly on the stack. Very gently bending the tabs to finish, and the switch can be placed back on the machine.
A small, cosmetic tweak - that somehow does make a difference for the whole appearance of the machine :)
(As practice for more cleaning on the Underwood 5 to fix a now-sluggish 'S' key, first started with cleaning the segment and type-bars on the reference machine.
Once you get the hang of it (but only then), type-bar removal and placing-back is surprisingly easy on the Underwood No. 5.)