To be able to make these changes, your user will need to have permissions to customise your CRM (the out-of-the-box System Administrator security role contains the necessary permissions).
In Dynamics, go to Settings > Customizations.
Select Customize the System, then expand Entities > Campaign Response
Open the Fields section, then select the field responsecode and choose Edit. This will open a new pop-up of the field settings.
At the bottom left, all available responses will be listed. To add another option, simply click on the Plus sign. The field will automatically be called item, but you can edit the label on the right side. You may also add a description of the response code.
Save and close to update the field. Once you have saved, publish all your customizations for the options to become available for all users.