You can customize the confirmation displayed after registration is completed by editing the Event Confirmation Message. You can find this message option on the Attendee Setup tab.

To customize an event confirmation message click the pencil icon in the Attendee Messaging section.

To customize the email the registrants receive after registration is complete,  use the dropdown menu to select a Confirmation Email Template you've already created, or select the plus sign to create a new one.

To customize the web page displayed after registration is complete, enter your own text in the Confirmation Message field.

The Confirmation Message is displayed on the registration confirmation page.

The Confirmation Message is also added to the Confirmation Email registrants receive following their registration if a custom Confirmation Email Template is not selected.