If you want to replace the left & right wheel complete.
Put one side on a block of wood, not too high! Look under where the wheel is and you will see on either side of wheel axel is 'L' shapped plate with two Allen screws.
Remove screws from both sides & the wheel shoud drop out.
When putting new wheel in you may have to adjust the footbrake. Adjustment mechanism is accessed from to the by removing the foot lever end Allen, you then have to tap the Hex axel out (it might be stuck) do not damage Hex Axel, one Hex Axel is out far enough to access the brake pluger (small ball bearing on top) and screws.
Ajustment is done with very small locking Allen, be carefull not the damage the plastic sleeves around plunger.