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.
If you would like to remove ALL attendees, check the box next to registrant, instead of next to an individual attendee, and then click on the trash can icon.
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.