How do you sing a song that is not in your vocal range?

either transpose it (if the original is in B for example sing it a few tones lower, like G or something) or learn to sing it the way it is. Transposing is effective as long as you have a backing track in the same key. That's why it's like a bonus when you can play an instrument (like guitar or piano mostly) reasonably well, because then you can just transpose the chords and play the new chords yourself; then you can do any key.