When you remove the freewheel you will see there are spacers along the axle between the hub and frame, this offsets the hub to make room for the gears. So, to center the wheel you will have to move spacers around to the other side of the hub. Re-dishing the wheel is how you re-position the rim to line up with the frame, which it won't after you re-space the rear axle.

To do these things you need to know how to overhaul a hub and true wheels/use a spoke wrench. If not, find someone who can teach you or take it to a bike shop.