What is the best vocal warmup to increase vocal range?

Something I did that helped increase my range was singing the vowels in scales (ee-ay-oh-oo), starting from the lowest I could reach and going up to the highest. When you hit one where your voice starts to falter, try to hold the note steadily. If you can't, try again on the next one down. It takes time, but doing this every day will eventually help your vocal chords get used to those higher notes. Just be sure to work your way up gradually so you don't strain. Good luck hitting those high notes!

Do an octave excercise,

using 'do, mi, so' [C, D, E on a piano] and go

C D E D C D E D C [the C one octave above] C [the one you began with],

and go up the scales that way.

So what you'll be saying is,

Do mi so mi do mi so mi do DO do. [the capitals means it's the high one.]

"Pants on the Ground"