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.
The one time I tried to disassemble a revoshift with the intention of saving, it broke some flimsy plastic flange and wouldn't work again.

So the safest thing is to replace the shifter with a trigger shifter or a pod shifter or even a friction shifter lever.

If you're feeling brave, you'll want to remove any grip from the bars. and pull the gear cable right out. Then undo the little hex bolt holding the whole shifter to the bars - might be a M3 or M4. At this point the grip-shifter should slide completely off the bars.

At this point you're looking all around it to identify the clips that hold the unit together, and they're all variable in location. It should unclip around the axis where the handle bars went through. This is where mine broke.

Once you can get to the back of the clear window, clean it with IPA or meths. Do note that the black coating will come off too, potentially making your viewport window much larger.

Reassembly should be a simple clip-together then reverse the order. Do consider using a new inner cable if you can - they're cheap and the old one may be manky already.