Yes you can, if you are are security level 3. To do so, click the "Edit" icon next to their name, and find the "User Type" field near the top. Simply change to the desired user type, and they will be changed.
If you are using the maximum number of Planner accounts permitted by your subscription level, you will not be permitted to change a Helper to a Planner. If a Planner is a master account or team leader, you will not be permitted to change him or her to be a Helper.