You can change your equipped skills by tapping on this book icon at the bottom:

It will bring up the skill menu, split between "CHARGE" skill and "SHIELD" skill:

You can switch to the desired skill by simply tapping on them. To switch to the "SHIELD" skill section, tap on the shield icon as shown here:

It will bring you to the "SHIELD" skill menu:

Selecting skills for "SHIELD" is the same way as "CHARGE" skills. The skill description will let you know what each skill does.

NOTE: Skills do need to be acquired through leveling your Postknight..