Go to the "Teacher" tab.
In the top right of this "Teacher" page, you will find two different dropdowns. The first allows you to switch between your classes. You will need the second one, the "Manage" menu.
You can use the "Copy Class" option to copy an existing class. First, you give your new class a name- make sure that it's different from the old class name to avoid confusion. Then, decide which elements from the original class you want to copy.
Users: your new class will have the same students and teachers as the original class.
Users + Groups: your new class will have the same students, teachers and groups as the original class.
Course material: your new class will have the same course material as the original class.
Course material + Tests: your new class will have access to the same course material, as well as the tests or exams as the original class.
Course material + Assignments: your new class will have access to the same course material, as well as the assignments of the original class.