Hi @diveexplorer,
There are different propellers that are intended to operate with opposite rotations. If you have three propellers in your vertical thrusters that are all the same then one of them should be swapped out (although it may not make a huge difference).
If you have the correct propellers (two clockwise, two counter-clockwise, placed on the diagonals) but one of them happens to be spinning the incorrect direction then you can just reverse the spin direction of that thruster in QGC on the Motors Setup Page (either manually, or by running the “automatic motor direction detection” process while the vehicle is in the water).