Cantilever brakes and removing rear wheel

[lunacycles]

In case you are inspired to take it on wrt adjusting the cable at the brake lever/handlebar...

On the right brake lever (which controls the rear brake), as the cable housing exits the lever, you will see (and have seen) the cable adjuster. The cable adjuster is simply a hollow bolt screwed into the brake lever, into which the cable housing (and cable) goes. To create more slack in the cable to allow you to release your rear brake, you simply need to screw the adjuster "in" towards the brake lever. this means turning it in a clockwise direction toward the brake lever.

Usually there is a slightly larger cylindrical knurled piece/threaded washer that bumps up against the brake lever, locking the adjuster and preventing it from vibrating loose. Before moving the cable adjuster, you will probably need to back this washer off the brake lever a bit, i.e., turn it a few turns in the counter-clockwise direction, moving it away from the brake lever. Once you have done this the cable adjuster is free to move and you can simply turn it towards the brake lever to create some cable slack. Try turning it two revolutions at first, and see if that creates enough slack to release the cable at the brake end. If not, keep going until you do have enough slack.

Once you have your rear wheel back in the bike and have re-attached the cable to the rear brake, you can re-establish the original cable tension by just backing off the adjuster (now turning it counter clockwise) the same number of revolutions as you screwed it in.

It is not hard to do but also not easy to describe in words. If you do take it to a shop, ask them to show you how to do this, as it is easy to show you in person, will take 30 seconds, and once you know, you will have the ability to do what you intended: change the rear tire/tube.