1.Turn on "USB mode":
Click "Settings-About device-Custom build version" 6 times,and click "Open"
2.Turn on "USB debugging":
Click "Settings-About device-Firmware version" 6 times to show the "Developer options".
Turn on "Settings-System-Developer options-USB debugging"
3.Connect the TypeC port to your laptop by USB cable.(Don't do this before above 2 steps.If you already did,replug the USB cable).
4.Go and choose "Settings-Connected devices-USB-File Transfer"
5.In PC, you can see the M8 disk and can transfer file.
By the way,if you only want to install an app,also can use adb command to install,