Editing a default page has two stages. Editing the default page and then setting the new edited version as the default page for that action.
To start go to campaign > Pages and click the default pages button.
Find the page you want to edit and click orange action button. You then want to duplicate the page.
This will then duplicate the default page and place it in the custom pages section.
Click the orange action button on the duplicated page and then click edit on the dropdown.
This will open the editor. You can then edit the default page to how you want it to look or what you want it to say.
The next stage is to actually set it as the default page. To do this got to the orange account button in the top right and click settings.
On the settings screen, there will be some orange text that says ‘Click to select the default pages’
This will open up the options to set the default pages in your account. Once you have done this, click update settings and then the default pages will update to the new ones.