To edit any Contract, first navigate to the Contracts dashboard, then click on the contract name you wish to edit:
This will take you to that contract's Contract Details page where you can view more information and make edits. Because Contracts are based on Terms, changing the start/end dates or value for a contract can be done by clicking on the Terms & Change Orders section, then selecting Actions -> Edit for the Term whose info you wish to edit:
You will then be able to edit the dates, the value, and the currency:
Click the Save button to finalize your changes.