Click on a case to open it in the right-hand panel.
In the upper left corner, you'll see the Assign me button and dropdown. You can click Assign me to assign the case to yourself, or click the dropdown to assign the case to another user. To the left, you'll see the profile picture of the user the case is assigned to, which will look like a question mark if the case is unassigned.
From the dropdown, select the staff member to whom you want to assign the case. An alert will be sent to the new assignee via email.