These improvements are automatic, as long as the requirements are met.
As your character's characteristics improve, the type of unarmed attack will change. ITEM2 covers using your feet (don't ask, it involves some weird yoga practice.) Your character starts by just being able to punch or kick, but there are more unarmed techniques than that. You must not have an active item in one of your slots to use this skill to its full ability, however. Very few weapons will also use this skill (the armed version of Unarmed), like Brass Knuckles.
