If an approver is not able to edit the expenses then please do the following:
Check the user is enabled to edit approvals click settings within expense and then approval.
Find the appropriate approval type and click into it.
Check the users is both set up as an approver and that the allow approver to edit box is ticked.
Going Forward newly submitted expenses to this approval type will be editable by the user.
If there are historical expenses that need editing these would need to be rejected and re-submitted.