Do you have the rights to change contracts?
If you're a User, you may not have permission to change details in a contract.
First, check your User rights by going to Settings > Manage users. The assigned permissions are listed at the bottom of the menu.

If your permissions look like the picture above, you do not have sufficient permissions to change details in contracts or templates. If you wish to change this, please contact your account Administrator.

Are you a participant of the contract?
If you aren't a participant of the contract, you can only 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.