You can change your watch buttons by navigating to the Customize Device icon section of the app and tapping “Edit” next to the feature name and assigned button. Choose a new function for the button you want to change from list of available features. Your watch will now perform this feature when the assigned button is pressed.
If you have already set a feature to a button, you will not be able to set the same feature to another button. You must first change the button setting the feature is originally assigned to before you can assign it to a new button.