How do you change speakers to headphone in PC?

Usually, you just plug in the headphones, and that mutes the built-in speakers. If the speakers don't mute, make sure the plug is the right size for the PC's jack (the plug should not have any wiggle when it is plugged in), and that it is plugged in all the way into the jack.
Unplug the speakers from the computer jack and replace it with the headphone jack