I haven't changed bike tires myself for many years. Always had the nack and even fitted my own car tires a few times. It's all down to the way you do it rather than brute force.
I put the tire in a big engineers or wood working vice to break the seal but don't scratch the rim. Get the the whole tire loose first then push it into the well as much as possible. Use a bit of soap and pull as much off the side. Lastly use a mallet to get the last bit off. Pull out the tube. Do the same again the other side so the wheel is between the two walls then just pull it out.
To fit the new one slip it on one side then fit the tube. Wipe off any soap. Put a bit of air in the tube then push the tire into the well again. Lay it on an old carpet and push it on by walking round the wall and digging with your heel. The last little bit use the mallet and maybe a tyre lever.
The secret is to do little bits at a time. Make sure the tube isn't trapped and clean off all the soap. If water gets in leave it to dry out before pumping up.
Simple as that.
Now leave me alone while I have a heart attack.
