It depends which advance care plan(s) you have setup. All allow changes but some have a more complex process than others.
Gentle Dusk Advance Care Plan
You can update your ACP by adding Addendum Page(s) then signing and dating your updates.
Co-ordinate my Care
Information can be updated but you need to resubmit to your GP before they can approve on your changes.
Advance Directive
While you still have capacity you can change your mind or cancel your Advance Directive at any time. If you want to change it, you should make a new form and give copies to the people close to you, your GP and anyone involved in your care (instructing them to destroy the old version). You can cancel your Advance Directive by destroying it (and any other copies you have made).