Or rename your existing notebook first to save your code for potentially “copy/pasting” later. But when you do that, be very careful (as Saif and Tom have said) only to modify the “YOUR CODE HERE” segments in the fresh clean copy.
But the key point to realize is that the “Get Latest Version” procedure will only replace missing files, which is the reason that you must first either delete or rename your notebook.
Also note that the grader will only grade the notebook with the standard name.
Here’s a thread which gives the full “tl;dr” details of this process.