My fleet just arrived to a system I was going to blockade and then assault with a small number of troops. But I occasinally misclicked and started assault on the first turn after arriving. Is there a way to stop assault and withdraw my people from that planet back to the ships to save them?

The ground battle will continue in the next turn, and you'll have to option to "Retreat". This brings back your remaining troops in your ships and you can leave the system if you so wish.

Thank you!