Yes, you can undo an expense approval as long as the expense is cancelled before the cut-off date. Simply click on Actions --> Change Review to change the status. The expense status can be changed from approved to declined or pending.
If you want to undo an expense reimbursement after the cut-off date, please contact our support team.
Also see:
Can an admin submit expenses on behalf of Team Members?
How do I add an expense approver?