The PX Action under category attributes does not allows to update/modify the attribute(s) of a specific user. It only allows to update the subject of the task. There should be an action defined , to allow the specification of user whose attributes needs to be updated. This will help in case where user's value need to be changed based on the hierarchy.
I'm referring to a post of mine which for better understanding of this and the need for this.
Update Attribute of a Specific User