For a purchase request in the Req Code box select “RW Restart Workflow” and save. If “RW Restart Workflow” is already selected, unselect it and save. For an invoice you need to make a change to the actual record, for example add a period at the end of the description. Then you should have the option to save and workflow will restart. Optionally, you could make a change save it, then revert to the original and save again.