How do you become left handed if you are right handed?

Answer 1

Practice. Do your normal stuff with your left hand if you are right handed when you've practiced enough.

Answer 2

You can't; handedness is inborn.

Revise: No, I believe you can teach yourself to become ambidextrous

but true handedness can't be changed and results from how you use each side of your brain which, unless you experience extreme trauma, is the same throughout your life and built in as a part of your personality.

Answer 3

Practice. Do your normal stuff with your left hand if you are right handed when you've practiced enough.