You can edit any available field on a Contract after it has been activated with one exception: Account Name.
In order to change the Account on which the Contract resides, you will need to delete the Contract and create a new one.
To delete an activated Contract you will need the "Delete Activated Contracts" Profile permission and "Read" permissions on the Accounts object (more about Deleting Contracts).
To delete a Contract, click Del next to the contract on the Contracts list page. You can also delete a Contract by clicking Delete on the Contract detail page.
NOTE: When you delete a Contract, all related Notes, Attachments, Events and Tasks, History, and Approval Requests are deleted. Associated Accounts are not deleted with the Contract. The deleted Contract is moved to the Recycle Bin. If you undelete the contract, any related items are also restored.