To download data for a project, go to the project home page and click on "View Data":
On the navigation bar at the top, click on "download" to open a left side panel with the download settings:
A panel will slide in from the left:
Select the mapping you want to use
(The default mapping is always pre-selected). More on mapping data
You can also select which timeframe you are interested in.The default is "LIFETIME" to get all the data since the project was created.
If you need a custom timeframe, select "CUSTOM" and pick the start and end date.
Click on the project zip file to download it.