By following the navigation prompt, you'll be directed to the following screen
When you're not the currently assigned facilitator, you'll be prompted with the following modal message
Click OK and you'll then be taken into the session screen where you'll be able to facilitate the session - scoring calibrated results, adding feedback and marking the participants' register.
If the previous facilitator had chosen to evaluate before the session, they will simply become a participant for the session.
If they had chosen not to evaluate before the session, the original facilitator will no longer be part of the session.