After the assignment opens up, click "File" (top left) and then "Open…"
When your workspace opens, select the check box before your assignment file. After it is selected, press "Shutdown".
Using the same procedure mentioned above, "Rename" your file. For instance you can change it from assignment_name.ipynb to assignment_name_v2.ipynb. By doing this you’ll be able to save your current progress on the assignment after the latest assignment file is fetched.
Using the same procedure, "Delete" any other file, if any, that you want to get a fresh copy of.
After renaming your file, click on the "Help" button on the top right of the page. From the panel that opens, click "Get latest version" button, and then "Update lab".
After the page reloads go to File → Open… , as described in (2)
Now you’ll see two notebook files. The one you renamed (as done in step 4) will have your previous progress, and the latest version. (Depending on how many previous versions you have kept saved, you could have more than two assignment files in the worksapce)
Option 2 - via URL
Follow the steps 1 to 5 as mentioned in Option 1.