Disable plugin function to use my own

You can use remove_action() to remove a function from a specified action hook. Documentation

remove_action( string $tag, callable $function_to_remove, int $priority = 10 )

This function removes a function attached to a specified action hook. This method can be used to remove default functions attached to a specific filter hook and possibly replace them with a substitute.

So in your case,

// Remove the function from 'save_post' action
remove_action('save_post', 'um_activity_new_woo_product', 99999 );

// Add your own function my_activity_new_woo_product(), to 'save_post' action
add_action('save_post', 'my_activity_new_woo_product', 99999, 1 );

I hope this may help.