Hover over the button, in the grey box click the gear/settings icon

Content Tab:

Button - change the text appearing on the button

Link - change the link location of the button

Design Tab:

Alignment - change the alignment of the button

Text - change the text style

Button - change the button style

Spacing - edit spacing of button

Box Shadow - add shadow to button

Filters - add filter to button style

Transform - edit button positining

Animation - change how the button enters the screen

Advanced Tab:

CSS ID & Classes - for custom styling through CSS

Custom CSS - for custom styling through CSS

Visibility - add if you want disable the button from appearing on a phone, mobile or desktop

Help Video: https://www.elegantthemes.com/documentation/divi/button/

Once you are done editing your button, click 'Save' to save your page changes