The Renew a contract screen opens with a new contract record that shows the details of the contract being renewed but with the next date period.
Make any necessary changes to the new contract and save. For more information on updating contracts see here
Once this contract has been saved it appears in the Contract Approval screen.
If a contract is not going to be renewed, exit back to the Contracts renewals screen scroll to the contract and click Don't renew.
To reverse that action and put the contract back on the renewals screen open Review contracts for the relevant contributor. Ticking the Renewable? box will put the contract back on renewals.