How do I make changes to a requisition that has already been submitted?

Problem

I need to add an attachment or make changes to my requisition that has been submitted.

Changes to a requisition can only made if it is Returned or Withdrawn:

You can access the PR Approvals tab to view the next pending Approver and request that they select Return to Requisitioner from the Available Actions.
You can use the Available Actions option to Withdraw the Requisition, then Copy to New Cart, make the changes and then Place Order.

If only an attachment needs to be added and there are no changes to the requisition

Attachments can be added to Comments on Purchase Requisitions and on Purchase Orders.