To edit any Contract, you will go into the Contract Details page first:
Once in the Contract Details page, you will be able to edit any part of it. Contracts go by Terms. So if you wish to edit the Start / End Term, Value, etc., you will go under the Terms & Change Orders section and select Actions -> Edit in the same row as the Term you wish to manage:
Here you will be able to edit the dates, the value, and the currency:
Click the Save button to finalize your changes.