Change the variable value in an other plugin function with PHP

I try to change a class, which is stored in a variable without making any changes in the plugin file. I’m not sure how to target the variable inside the function.

This is the function: function learndash_mark_complete( $post, $atts = array() ) { Inside this function, there is a variable called $button_class.

if ( isset( $atts['button']['class'] ) ) {
    $button_class = ' class="learndash_mark_complete_button ' . esc_attr( $atts['button']['class'] ) . '" ';
} else {
    $button_class = ' class="learndash_mark_complete_button" ';
}

Is there a simple way to change the code to..

if ( isset( $atts['button']['class'] ) ) {
    $button_class = ' class="myclass' . esc_attr( $atts['button']['class'] ) . '" ';
} else {
    $button_class = ' class="myclass" ';
}

without change the plugin file. Maybe to create a function in functions.php or in a seperate (new) plugin?

I saw articles on the internet about add_filter, but I’m not sure how to target the part I need without wipe the whole function.

I know I could change the class with jQuery or something, and maybe it should be a lot easier, but I like to learn new things.