Do you have the permission to change contracts?
If you're a User, you may not have permission to change details in a contract (unless you are a participant in the contract of course, if so see below). You need to have the Member Role of Manager or Contributor in a Collection to have sufficient permissions. Please check with your Administrator.

Are you a participant of the contract?
If you aren't a participant of the contract, you may be able to view the contract but not make any changes to it. If you wish to make any changes, ask your colleague who created the contract to invite you as a participant.

What role are you assigned in the contract?
Only Signatories and Influencers can interact with a contract. If you're a Viewer and wish to interact with the contract, ask a Signatory or Influencer to change your role.

Is the contract marked as Signed?
No changes can be made to a Signed contract.
If you wish to make any changes to a signed contract, make a copy by clicking "Copy contract" in the lower right corner of the application.

Make your changes, save them and send them to your counterparty for a new signature for your new contract.