In order to Validate Invoices in bulk at a time, the user can choose to follow the steps below: 

If the user wants to validate multiple invoices in a single go, select purchase menu and then invoice validation. A new screen will appear, as shown below.

Now, select all the records that you want to validate by check-in the check box present at the beginning of each record or by clicking select all option present at the top of the list. Once the selection is done, click the validate invoice button at the top, and then a new screen popup will appear, as shown below.

You can enter the valid invoice number and invoice date here, then click the save button to finish the invoice validation process for all the selected records.