In order to change the view in Scoreboards, you first need to be on the Scoreboards screen. You can navigate there by going to the main page and selecting Insights and opening up the Scoreboard.
Steps to View a Students Scoreboard by Roster or Grade
Go to Insights
Go to the Students Scoreboard
Click "All Students" to access the Roster Picker
Choose the Roster or grade you'd like to focus on
The Students Scoreboard will then only show students in the Roster or grade you chose.