Go to Resource Planner > Find the class
Click on the group activity > Click on the "pen and paper" icon to view the booking
Go to the "Base information" tab
Click on the instructor under "Resources"
Click on the arrow down next to the instructor to choose a new one
Click on "OK"
If the instructor you wish to switch to is not in the list, it is because you didn't add this instructor to the group activity product under "Resource requirements" in Configuration. See the article below on how to add this to the product: