The approver for an employee is the person who first receives notifications for any requested absence. If the employee moves teams or departments, you can change the approver in two ways.
The quickest way is to go to "Account" and then "Employees" and click on the little arrow next to the current approver's name. This will show a drop-down menu of all other approvers on your account (if you don't see the name you want, you will need to edit that person's record to make them an approver).
Change to the correct approver and the box will flash green once the update is processed. If you are making a lot of changes to approvers, this is the quickest way to make them.
The alternative is to go into the employee's record and edit the approver from there, which may be quicker if making multiple changes to one employee's record.