Renaming Bluetooth device not working [duplicate]

Of course there is a .plist file to (backup and) delete.

1. First of all close your Mac's Bluetooth
2. Open Finder
3. From toolbar select -> Go -> Go To Folder
4. Copy paste this path /Library/Preferences
5. Locate the file com.apple.Bluetooth.plist and rename it to com.apple.BluetoothOLD.plist
6. Then do the step 3 again and
7. Copy paste this address ~/Library/Preferences/ByHost
8. Locate a file that starts with com.apple.Bluetooth... and rename it.
9. Restart your Mac
10. Re-enable your Bluetooth and try again

Use the Bluetooth Debug Menu:

This is your solution of last resort, if you use this method your Mac will delete all of your Bluetooth devices and you will need to reconfigure all of them again.

To use this solution you need to have the Bluetooth icon in the menu bar (and as I saw from your screenshot you have it).

1. Press and hold OptionShift key and click the Bluetooth icon.
2. From debug option, choose first Remove all devices and then Reset the Bluetooth module
3. As always "restart your mac" and try again to pair and rename your Bluetooth device

More information can be found here.