Change your voice by imitation. Choose a role model, listen to how he speaks, and start copying the way that person sounds as he talks. Practice by talking along with a recording of that person speaking.
Listen to your role model. Use a recording to listen so that you can play it repeatedly.
Repeat the words your role model says. Pay attention to this person's tone. You may need to physically change the way that you speak. Keep practicing.
Record your voice with the dialog that your role model uses. Ask yourself if this sounds the same. Repeat as needed.