To amend the activity field in the case history, follow the below steps:
Go to User Profiles in the Authorisations tab.
Under the History Screen section, select the option to allow users to amend item activity.
After you've enabled this, ensure the Activity column is visible in the Case History. Right-click the relevant history entry and select "Amend Activity" from the menu that appears.
Note: If the Activity field is greyed out in User Profiles, this needs to be updated by the Default user, please raise a new case online and reference the title of this article.