To extend the expiration date of a submitted quote, you will need to execute the following steps:
Hit the Revoke button of the contract page.
The following message will appear:
Hit Proceed.
The stage of the quote will now go back to Draft. Click the Edit button of the properties section of the quote and change the number of days in the field Due days quote. The field Expires on will automatically be re-calculated.
Once you are done, hit Save.
You will now be able to publish your quote once more.