In case you want to change the appearance and functionality of appointment form and appointment data in customer account, you can very simply override plugin template files with your theme. Process is the same as with default WooCommerce plugin, directory name of edited template go inside woocommerce/. Learn more about overriding template files with a theme here.
Where are template files located?
Templates files for Appointments can be found inside the directory:
/wp-content/plugins/woocommerce-appointments/templates/
Placing new template files inside your theme
To edit these files, place them in the directory:
/wp-content/themes/your_theme_folder/woocommerce/
An example
If you want to edit the my-appointments.php file in the myaccount template directory, you would place the file in:
/wp-content/themes/your_theme_folder/woocommerce/myaccount/my-appointments.php
Do the same for all other files.