I am guessing that you may be having trouble releasing the bolt that holds the wheel to the shaft. This is usual to be tight after use due to the water and muck around the threads. A set of mole grips firmly holding the wheel will stop the wheel from rotating as you apply pressure to the centre bolt on the motor shaft. Failing that, with the wheel locked someway and a good fitting spanner or socket on the centree bolt try tapping the end of the spanner with a hammer. This usually frees the bolt.
When replacing a wheel i find that a heavy cloth placed around the wheel and a firm grip is all that is needed to hold the wheel while i tighten the centre bolt.