Talk more.

Game walkthrough comedian Markiplier had his voice deepen considerably over the past couple years.

https://www.youtube.com/user/markiplierGAME

Don't concern yourself with his CONTENT; find a video from 2 or 3 years ago of his and listen to his VOICE...then listen to a recent video to hear just how much it has changed.

You obviously don't have two years. But merely by talking consistently will exercise your vocal cords and result in a deepened, stronger voice.

A vocal coach is a paid option.

Youtube searching for "how to improve my voice" came up with a bunch of video results.