First, I wanted to get the three front gears working. To do that, I wanted to replace the cable, but I can't see how you do that in the shifter I have. It's a combined gear shifter and brake, and I decided to replace it with two separate units. I already have these on my box of bits.
The inner cable of the shifter looked slightly rusty, so I gave it a good stroking with emery paper, then oiled the inside of the outer, and it ran smoothly. I installed the new shifter, and it worked! I can now change gears on the front sprocket - I don't want to do this very often, but it does help when going up a steep hill.
Next, I wanted the new rear gears, which go as high as 11 teeth (previously, I could go as high as 14). First, I taped the inside of the wheel. Then I put the tire on, on one side. I put a bit of air into one of my very thick puncture-resistant inner tubes, and pushed that onto the wheel. Then I added the gel insert, that sits between the tire and the inner. All this is to avoid a thorn giving me a puncture, becuase I ride on rough grass, and I'm often going over brambles and blackthorn. Finally, I put the tire on fully, and I was surprised how easy that was - no tools required, and not much thumb pressure. I inflated it up to 45 psi, and now it was ready to go on the bike.
I turned the bike upside down, and took off the back wheel. This is the wheel that iuntil recently, had been my spare, and it was going to become a spare again. The new wheel went on easily. I've never used a wheel with quick-release skewers, but it was obvious how to do it. One benefit of this - there's a spanner I dn't need to carry with me in future, saving a few ounces of weight.
The back gears needed a very slight adjustment, ditto the back brakes. I oiled the chain, and it all looks good.
So now I have an electric bike with a two-speed motor, I can use an 11-toothed rear gear for when I'm going fast along tarmac, and I have access to very low gears for when I'm struggling up a steep hill. The brakes both work, the bell goes "ding" and I even have a clock/thermometer attachment!