1) Select all cells (or cells that you need) in your read-only notebook. You can select all cells by clicking on the first cell and then shift+clicking the last cell.
2) Copy all cells using CTRL+C (COMMAND+C if you are using MAC)
3) Create a new jupyter notebook page

4) CLICK IN THE MARGIN OF THE NEW CELL (***to close it for editing***), then...

5) Click CTRL+V (COMMAND+V if you are using MAC)
6) Save your new notebook