Once your webinar has been created you can find in the Registration section while editing your webinar.
When you created the event, a default registration page was created for you automatically with placeholder contents. You can go to "Registration" and then edit the content by clicking here
If you want to change your registration page, then click here
Please note if you change your template, all your contents that you might have edited for the Registration Page, will be lost and they cannot be recovered.