You can edit the emails your guests receive during the self check-in process here.
Note: Same process can be used to edit the confirmation email.
Merge tags are used in the default text to provide elements such as the event name and a signature line "out of the box" but when editing, it is easier to replace these with plain text than to figure out where to edit the source.
To edit self check-in emails:
Go to Gala Auction > Self Check-in > Click on Edit Invitation button
(shown) Admin > Site Settings > Communications > Email Templates > Invitation to Self Check-in