League of Legends (LoL) Question: How to play Nasus top when outnumbered?

I would recommend going to a solo lane when you can't farm top, i.e. bot. Try and get your team to peel by pushing a lane so the jungler is forced to go to them to stop them from getting objectives. If you're jungler is way ahead ask them if you can take farm for stacks or join in team fights to get stacks off of kills, if the enemy teams jungler isn't around you can also get stacks from objectives, there's a lot of ways around this. If it's early game and you're being caught under turret you can ask for a counter gank and try to secure a kill or if you're not able to 2v1 tell your jungler to try and hypergank another lane that way the enemy jungler will have to do something about that, or even just tell them to do an objective because even while being weak or pushed under turret early you have massive scale and you'll still be able to CS throughout the game.

If you're aware that the enemy team is trying to go top and kill you, I would advise to play safe as you are currently playing the weak side of the game and so the jungler will be less likely to help and might instead try to get botlane ahead.

If you think they'll dive you, I would just leave the tower until the enemy jungler is gone or if you can't defend it at all, then leave and let them take it. Otherwise you're sure to die as Nasus isn't exactly strong in the early phases.