What you may not know is how they do it. Tree squirrels have ankle joints that are adapted to be super flexible. When a squirrel wants to climb down it can rotate its feet 180 degrees, dig its claws into the trunk and hang from its back legs.
In human terms this would mean that you could rotate your feet around till they were pointing backwards.
Sometimes squirrels will even hang on by their back legs and swing like a bat. Our backyard squirrels will often do this in the morning, probably to stretch their backs.
Also squirrels have very sharp claws (which is a good reason you should be very careful if you try and feed a squirrel by hand!) and when they dig their claws in with all four legs they can get an amazingly strong grip on a tree limb.
They can even climb along the underside of a branch.