Once a contract has been signed, you won't be able to make further changes to the signed contract.  However, you can always make a copy of the contract, make your revisions and resend the updated contract for signatures.

To make a copy, navigate to the contract and click on the gold action button towards the top right of your screen.  Select the Create Copy option from the menu, edit as needed, then follow the steps above to rename your contract (for example, you may want to replace copy with Version 2), add document signers and resend.