The user has a PO that has been received, but the supplier on the PO is incorrect. How do you make sure that the correct supplier is paid?

The best way to update the supplier is to change the supplier when you enter the invoice. To do this, go to Accounts Payable > Invoices > Invoice Entry, enter in your Tran ID, and you can then update the supplier on the invoice using the locator next to Supplier ID. 

If you have already entered in the invoice, but have not paid it yet, you can go to Accounts Payable > Invoices > Invoice Edit and enter in your voucher number and update the supplier on the invoice here.