Can I change who my buddy is?

According to this report,

Buddy Pokémon can be swapped out at will in “Pokémon Go’s” latest update, but doing so has no impact on your total kilometers walked stat. It will, however, reset progress toward your next candy. Just make sure all incoming candies are earned before making a switch.

So, the answer is yes, but the progress towards your reward will be reset each time you switch.

To change your buddy, follow these steps:

Click the button on the bottom left of the screen showing your character’s face.

Hit the button on the bottom right of the screen with the three lines.

Select “buddy.”

Hit the button on the bottom right of the screen with the two arrows.

You will be asked “Are you sure you want to swap your buddy?” Hit yes.

Select the Pokemon you want to have as your new buddy.