Purchase order status stuck to In-Process

Dear,

I have made a new position hierarchy but forgot to assign it in approval assignment. user has sent purchase order for approval and approver has received the notification to approve but when he approves that document it doesnt change the status of the document. PO is still in process.

I realized my mistake and assign the position in Approval Assignment now. but is there anyway to re-process the approval for that Purchase order which is still in process?

Regards