What can be updated depends on:
When an agreement is in Prepared status:
Once the agreement is signed, certain restrictions apply in function of the type of agreement.
For other types of agreements like contract and credit card tokens, we only allow the language and agent to be updated once the document is signed. See Credit card expiry and renewal options for what happens to a credit card agreement as it approaches expiry.
When editing, you may see up to three tabs.

Contains the core fields of the agreement, such as reference, IBAN, language, and agent. Which of these can actually be changed depends on the agreement's type and status — see Type of agreements above.
Allows you to update:
Visible if custom attributes are defined in the profile settings.
You can manage custom values linked to the agreement.