Hi there. Thank you for reaching out. To change the username on the DJI Fly app you need to go to Profile> Tap your username> Edit. You may refer to the image below for your reference:
Currently, Android devices do not support changing the distance unit for flight records. The default display unit is M or KM. The DJI engineers have been informed of this issue and will address it through a future app update. This will not affect flight records. Feel free to continue using your product. Thank you for your understanding and support.