Question: Shimano revoshift is fogged so gear number is not readable. I’ve washed the outside with isopropal to no effect. How to remove lens and or replace it ?

Answer: 
It might be possible to remove, clean and replace the cover. As Carel says, know you can replace the part before disassembly.

The [Dealer's Manual for the REVOSHIFT Shifter](https://si.shimano.com/pdfs/dm/DM-SL0002-03-ENG.pdf) seems to indicate the method for removing the cover.  

Page 14 tells how to remove the cover. 

[![enter image description here](https://i.stack.imgur.com/HxDfm.jpg)](https://i.stack.imgur.com/HxDfm.jpg)

Page 15 tells how to put it back on).

[![enter image description here](https://i.stack.imgur.com/OgPrF.jpg)](https://i.stack.imgur.com/OgPrF.jpg)

I left step 3 out for brevity. It tells how to remove and replace the cable.

I'd be tempted to leave it alone and shift by feel.
You've not mentioned how long it's been like that. If the moisture appeared after a very wet ride then it might dry out reasonably quickly and not come back until another equally wet ride. Stating the obvious is allowed on stack exchange isn't it?