Change the Description field of an issue in Redmine
To update the "Description" field of an issue, display the issue and click "Edit". Then click the pencil icon on the right of the "Description" label. The page changes so that you can edit the description.
To edit the "Description" field of an issue, you need the "Edit issues" or "Edit own issues" permission. By default, the "Edit issues" permission is given to the "Manager" role and the "Developer" role. You can check and change the permission assignments on "Administration" → "Roles and permissions" → "Permissions report".

