Every session in your study calendar carries a colour for the team member running it, so you can see at a glance who's taking what. To reassign a session, click it and choose Assign to.
How the colours work
Your own sessions always show with a light blue bar near the left edge.
Other team members are assigned colours automatically as they're added to sessions.
The legend is in the top right of the calendar view: each team member's avatar appears with their colour.
Reassign a session
Click the session in the calendar.
Select Assign to.
Choose anyone on your team to take the session.
Session assignment is separate from the study's facilitator setting, which controls who participants see as their contact. See Difference between organiser and facilitator.