The way the function is written, the default values are not editable from within the plugin as there is no filter hook to alter them.
However, this is a specific “attribute” of the shortcode. shortcode_atts() is a function that defines the shortcode’s attributes (which can be passed through the shortcode itself) and sets their default values. The default is only used if the attribute is not specified in the shortcode. When specified, this passed value of the attribute will be used instead.
So if you want to pass a different class, you would do it when using the shortcode:
[name_of_shortcode class=”something-else”]
(Note: I used “name_of_shortcode” since you did not include the specific tag of this shortcode – edit accordingly)