First you must select the tools button and click on System Configuration.
Once in System Configuration, select a user within the system by either typing into the search tab or expanding the lists below.
After finding the person you'd like to change the approval systems with, please do the following :
1. Select their name
2. Click on edit
3. Go to the approvals tab on the right hand side of the screen
4. Select the type of approval you'd like to give them from the drop down tab
5. Hit save