Note: Only the Course Admin can assign a new Lecture In-charge, and this can only be done till the scheduled start time of the lecture. If you are a Lecture In-charge but you won't be able to teach a particular lecture, you can talk to the Course Admin to assign the lecture to another course instructor.
As a Course Admin, here's how you can change the Lecture In-charge -
On the Course Timeline, navigate to the lecture whose in-charge you need to change
On the Class Page, navigate to the Agenda tab
Use the "Edit" link adjacent to the Class Team section to select a new Class In-charge
Only a registered course instructor can be made a Lecture In-charge.