In that case start with the front wheel first to gain confidence Andy, as it's much easier, but I'm not sure if you mean the Marathon Plus or the tyres I'm sending. Those are the standard Kenda puncture resistant ones and may be the same as you have. If you are changing them, here's the procedure. You'll need some tyre levers and a pump.
Turn the bike upside down, grasp the curved metal guide on the front brake cable and pull it outwards to unhook it. You may need to screw in the adjuster at the lever to give a little slack. Now loosen both front wheel nuts sufficiently for the wheel to come free and lift it out.
Now remove the dust cap on the tyre valve and press in the centre pin to release as much air as possible. At the opposite side to the valve, work the tyre into the centre of the rim to maximise it's slack on the rim, then insert the curved end section of a tyre lever adjacent to the valve and lever the tyre off the rim at that point. Be careful to avoid pinching the inner tube as well. The Tyre lever will have a slot at the end of the handle part, so hook that onto a spoke to keep it in place.
Now insert a second lever just the other side of the valve and lever off again, repeating the hooking as before. then with a third lever start a little bit further on from the second and repeat that operation. At this stage the tyre off of the rim should be slack enough for you to start pulling off all of that side, using the first lever to help that if necessary. Once that side is fully off, pull the tube out from the tyre, ending wit the point where the valve is. Pull the tyre back over itself at that point so that you can lift the valve out of the rim, leaving the rim tape in place as that protects from the spoke nipple ends. Finally pull off the second edge of the tyre, called the beading, off the rim.
Reverse the procedure to fit the new tyre, making sure the tyre direction is correct as marked on the side by an arrow or similar. The arrow should point in the direction of the bike's rear at the road surface point. Start with one beading, then fold back the tyre to insert the tube's valve, and with a touch of air in the tube, not too much, to give it a little shape, place the tube back inside the tyre, making sure it's not twisted at all. Then the second beading can be worked back on, starting with the side opposite the valve and squeezing the tyre towards the centre as you go round evenly both sides to end up near the valve. Push the valve stem in a bit to make sure it's sitting inside the beading, then finally pull the rest of the bead over the edge of the rim. You may need the levers to do that, but this time again make sure you don't pinch the tube as that could puncture it.
Again push in the valve just a little to make sure it's sitting within the tyre, then "work" the tyre on the rim to make sure it's sitting evenly all round, judging by the circles on the tyre walls near the rim edge that are provided for this purpose. Now pump the tyre, slowly at first to ensure it sits evenly, finally bringing it to full pressure. Replace the wheel in the fork dropouts and tighten the wheel nuts evenly to secure it. Replace the brake cable that you disconnected, turn the bike back over and operate the brake lever a few times to align the brake. If necessary reset the cable adjustment on the lever.
Job done. As this will be your first time, that's enough for now, best come back when you are ready and feel confident to tackle for the more complex rear wheel details, which involve motor cables and associated parts.
P.S. I've just seen Ian's good advice in addition which will also be a very helpful source to learn all aspects from, but I'm happy to follow with details on the rear wheel as the manual is not exactly fulsome in this respect. I've had the Quando II's rear wheel out several times, changed tyres and tubes, and also stripped the motor so can give any advice necessary.
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