Excuse me, in which folder is the original SMS ringtone of lg gm75 put?

Just put it in C- My Documents-My Ringtone, and then set it in Settings.

Oh, the ringtone of the original phone is in the C:WINDOWS directory, but you need to copy it to your mobile phone, and then rename it and overwrite it with your mobile phone, otherwise you will be prompted that the permissions are insufficient.