How do I download/save the code I've written?

You can download the code you wrote in missions regardless of the subscription plan you're currently on.  For example, if you did some missions as a Premium subscriber, you would be able to download the code from those missions even after you cancel your subscription.

To download your code: 

Go to your  profile page using the menu in the top-right corner of your screen.
Click the 'Download Code' tab in the right panel.
Click on the course name that you'd like to download code for.
Save the  .zip file on your computer.
Unzip the file to access the  .py files containing your code.
ⓘ Downloaded zip files do not include the mission's instructions nor the dataset used in the mission. You can download the dataset by following the instructions here, but mission instructions cannot be downloaded for copyright reasons.

Downloading your code from a Jupyter Notebook

Downloading your code from a Jupyter Notebook requires a different set of processes, but is just as easy to do. From your notebook, you'll want to click on File -> Download As -> .ipynb which will download the .ipynb file to your local drive.