No, but as long as a Purchase Order has not been fully approved, its Requester has the option to edit the request (this means the Purchase Order will be resubmitted for approval) or to cancel it.
As soon as a Purchase Order has been fully approved, it can no longer be changed in ApprovalMax.
If changes are inevitable, however, authorised users are able to make them to the Purchase Order in QuickBooks Online. Unfortunately, changes won't be synced back to ApprovalMax.
Alternatively, you can create a new Purchase Order and ask to have the previous one rejected/cancelled/deleted.
If there have only been changes to the Purchase Order amount to increase it, it's recommended to create a new (additional) Purchase Order for the difference between the two amounts. This will avoid changes in the accounting system.