As long as a Sales Invoice has not been fully approved, its Requester has the option to edit the request (this means the Sales Invoice will be resubmitted for approval), or to cancel it.
As soon as a Sales Invoice 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 Sales Invoices in Xero. Any alteration will be synced back to ApprovalMax to track all changes made to a finance document in Xero after it has been approved in ApprovalMax. Also, the Organisation Administrator will be notified if the "Fraud detection – changes after approval" setting has been activated. You'll find more details here.
Alternatively, you can create a new Sales Invoice and ask to have the previous one cancelled/deleted in Xero.
If there have only been changes to the Invoice amount to increase it, it's recommended to create a new (additional) Sales Invoice for the difference between the two amounts. This will avoid changes in Xero.