Fitbit's interface only lets you export one month of data at time. This Python script exports all months.

You'll need:

• Selenium (pip install selenium should do it).
• An empty Downloads folder (there will be one CSV file for every month of data the script retrieves).
• Chromedriver on the same folder your script is (or provide the full path to it on line 11).
• Tweak lines 6, 7, 8 to provide your e-mail, password, and how many months back you want to go.