Once you are in the broadcast studio, you will see the below screen:
On the right hand side you will see the "Interface tab".
From the interface tab you can control how your grid of speakers and presentations will look like, based on four options.
When choosing the option to have one key speaker (e.g. option two) you can "set to primary" the speaker you would like to be the key speaker by hovering over their video feed and selecting "set to primary" that speaker (as you will see this also gives you the option to mute, remove (or add) the speaker to the stream).