As a client you have the ability to make amend an active contract.
Go to contract page and click on the 'Amend' button located below the contract timeline on the right side.
Click the the blue 'Edit' link on the section you want to amend, make your changes, click on Save and then "Apply Changes".
That's it! Now, both parties will need to re-sign the amendment. An email notification will be sent to notify the contractor about the changes and will be asked to re-sign.
You can cancel your amendments simply by clicking on "Cancel amendment", located below the contract timeline on the right side.