I accidentally vacuumed dirt in my pool using backwash mode instead of waste mode. How do I get the dirt out from under the sand?

There wont be any dirt under the sand. When you vacuum your pool you should do it on filter mode Otherwise you will lose a lot of water for nothing, this would be especially wasteful in the case of a salt water pool. Ofter having vacuumed the pool on filter mode and if the pressure gets too high backwash. after backwash rinse for a minute, then return to filter.