3. What should I do if the RETRO NANO is abnormal?

[Clear the pairing information on your phone]
Go to Settings-Bluetooth in mobile to delete the RETRO NANO pairing information and reconnect it to RETRO NANO. 
[Clear the pairing information on RETRO NANO]
When it is on, hold both the volume- and volume+ buttons at the same time for about 5s. (Valid in any modes)