There are two ways to move seats.
[Move to another seat]
You simply just walk and hop on to another seat.
This could be bit of a hassle compared to using shortcuts.
Press the [F] key to get off the vehicle and walk to the desired seat position and press [F] key again to get back in.
Use shortcuts to move to another seat while you're seated.
Ctrl+1 = Driver's seat
Ctrl+2 = Passenger's seat
Ctrl+3 = Rear seats
If you desire to move to the sixth seat, simply press the [Ctrl] +  on your keyboard.