This feature is only available in the legacy version of the app: Preorder Pro + Back in Stock.
To customize the look and message of the Notify Me button:
In the app, click Settings from the left-hand menu.
Click Notify Me in the dropdown.
Under Button Display, you can edit:
Button text
Button color
Message position
The message shown on the button
Scroll down and click Save on the bottom right.
Your changes will apply automatically to all products using the Notify Me feature.