Hi Saravanan,
From my experience you can't update a purchase requisition once is closed as standard functionality as this will have financial transactions.
You can update the PR if on a status of draft or approved by recalling the workflow and sometimes this is tricky.
The all purpose of the workflow functionality is to review the process and make sure that business processes are correct, so if you think you will still have errors after this approval I would suggest to create a second approval on the workflow.
If you still want to break the link, discuss the requirement with a developer for a possible customization.
