“I guess I will pull in some anatomy. Your Larynx, which is where the vocal folds sit in your body, is full of muscles that help control the vocal folds. Like all other muscles in the body, you can works to stretch and gain flexibility. Like Jefferey said, it doesn’t happen overnight. If I wanted to get into the splits, I’d work everyday stretching to slowly get there. If you want to extend your range up, do vocal warm-ups that push you up in that range, stretching the muscles.
Here’s the kicker though! Your lower range doesn’t work that way. You can strengthen you lower range by relaxing and using proper technique, but it is almost impossible to extend your vocal range because that is based off of your genetics and age. The low range of your voice is all centered around the thickness of your vocal folds. The thicker they are, the lower you go. As you get older, they thicken… other outside factors contribute to them thickening as well (allergies, heartburn, smoking, etc.) – tamara_hope