Thank you for the nice review.
I assume you’re talking about the “If you have shopped with us before, please enter your details below. If you are a new customer, please proceed to the Billing & Shipping section.” text before the registration field.
The WooCommerce Multi-Step Checkout plugin inherits the registration form from the WooCommerce plugin, so this needs to be changed within WooCommerce.
There are two ways to change it:
1. open the WooCommerce /wp-content/plugins/woocommerce/i18n/languages/woocommerce.pot file in PoEdit and change the text. Check out this article for explanations.
2. overwrite the “templates/checkout/form-login.php” template, as described here. You need to copy the /wp-content/plugins/woocommerce/templates/checkout/form-login.php file to /wp-content/plugins/your-theme/woocommerce/templates/checkout/form-login.php and then modify the text.
Either way will work.