Why does my T/S read Updating Software for two days now

Unplugging it could easily brick it... So I can only offer opinions on this.

If your in warranty, I say you should try unplugging it and in the unlikely case it bricks it, send it to nest.

If you aren't in warranty, then I advise you call nest and ask them what to do. If they tell you to try to unplug it, tell them if it bricks it it's on them.

I wouldn't imagine it would brick it. Not in a stage of the update where it could get stuck. The choice is yours.

Hope this helps.