TT or MegaSTE keyboards are not sealed, but if a key is actually bad, the entire mylar surface generally has to be replaced. You cannot just pop a key off and replace it since the mechanism doesn't work that way. You can disassemble the unit and try cleaning the contact or testing the mylar trace for continuity, but it is hard to reliably repair. I have attempted this with a conductive pen or by weaving metal threads in and out of the failed traces, but it didn't last. Please note that if you decide to open the keyboard, there will be many, many tiny screws to remove, and be careful when finally opening it, since the individual clear rubber key contacts tend to fall out and disappear somewhere.
Best Electronics probably has replacement keyboard mylars in stock. http://www.best-electronics-ca.com/