Overwrite the form-checkout.php template by the theme
The MultiStep checkout plugin does not allow you to override the form-checkout.php template by theme. This is because being a customized template if your theme uses one its the plugin stops working.
Many users, however, would like to copy the template from the plugin folder and copy it to your theme and then customize it.
To override the template so you need to blow up the control that we carry out in the plugin by adding the following code in functions.php:
add_filter ('yith_wcms_load_checkout_template_from_plugin', '__return_false');
From now on it you will be ignored the plugin template and loaded one present in the theme. Such customization is valid ONLY for the form-checkout.php template, other templates can override normally without adding anything in the functions.php of the theme.