As the Event Owner and Event User you have the ability to change a registered attendee's selected sessions anytime before the session begins (those sessions that they selected at the time of completing their registration process).
1- Log into Expo Pass with your credentials, https://login.expopass.com/login, and choose the appropriate event.
2- In the "Sessions" section, select the session that you would like to remove a registered attendee from.
3- Click on the blue box on the far left that indicates the # of current registrants for this particular session.
4- Select the individual that you want to remove from a session and then click on the trash can icon on the far right.
5- A pop-up warning will appear, just to be certain that you want to delete this registered attendee from this particular session. Click "DELETE"
6- Congratulations -- you have now removed this attendee from this Session.
If you would like to "ADD" a registered attendee to an existing session, please review this article: Adding a Registered Attendee to a Session.