Using Mission Planner
Connect your aircraft to the ground station computer using a micro USB cable
Launch the Mission Planner Program
Open the Mission Planner Flight Data screen
Select the “DataFlash Logs” tab on the
Click the “Download DataFlash Log Via Mavlink” button
Select the log(s) you want to download from the left pane of the popup window
Click “Download Selected Logs”
Wait for the log files to download
This will save that log to your MissionPlanner/logs directory, in a folder named after the vehicle type, such as OCTOCOPTER.