How can I end my contract with a client?

You can't end a contract with a client directly. It must be the client to do terminate the contract.

If you want to end a contract, you should contact the client using our messaging system.
You can also message the client directly via your My Jobs page - click on the contract, and you'll find all the information about your job, including the "Messages tab", where you can contact your client and let them know about your intentions.