In just a few basic steps you can modify a registrant for an event based on their email address.
Navigate to Events > View > Events.
Hover over an Event and select Event Reports > Manage Registrants.
Select a sorting option. Email address is the default.
Check Group By Primary if desired.
Click modify on the registrant to be edited.
If your goal is to remove someone as an attendee, click Un-Register.
When planning an event, proper organization is a major key to success. Knowing how to maintain your registrant information is a major step in the right direction for a well managed event.