From your Event Dashboard, go to Setup > Registration > Confirmations under the Setup heading.

There are three headers: Default (Attending), Not Attending, and Registration Closed. Click each of these headers to update the confirmation text.

Please note: for the Default (Attending) and Not Attending confirmation messages, you can customize the message that appears on the confirmation web page and the confirmation email.

Click Update in each section to save.

If you have a Business or Enterprise plan, you can also customize the Email Advanced Settings, specifically the header text, from name, and logo.