To change a registration, you must cancel the transaction
This expains how to manage a change request or cancellation in Event Manager.
Member Solutions - Event Registration - Welcome

Go to the Event Manager website and login.
https://events.membersolutions.com/show.asp?content_id=10671
Go to list of Forms.
At Forms Tab, click on the Registrants button

At the Registrants list, View Transaction for the person in question.

The icon is a little green $.
A pop-up will be generated for the transaction in question

To cancel the transaction, click the Process refund/cancellation button.
The pop-up changes to the "Are you sure?" window

1) Check the box.
2) Click the Process request button