Can I take my seat back?
That depends. Why are you going to change your seat?
I'm sitting in the wrong seat. This is my seat.
I can see how you would think that it is your seat. It's rude to make someone move if you left and come back. Unless that seat was at your desk or had your name on it, you shouldn't claim it as your own.
It will be fine, I'm going to get it back.
Just wait till next time. You cannot make somebody move after they already sat down. Next time just leave something on the seat so somebody knows that there is somebody there.