Hi,
you can change the “If you have shopped with us before …” text by copying the plugin’s /wp-content/plugins/wp-multi-step-checkout/languages/wp-multi-step-checkout.pot file to /wp-content/language/plugins/wp-multi-step-checkout-en_US.po (or use your language’s locale instead of en_US, if necessary, see here a list of locales) and modifying the translation for the “If you have shopped with us before …” string in the wp-multi-step-checkout-en_US.po file.
You’ll also need to compile the wp-multi-step-checkout-en_US.po file to wp-multi-step-checkout-en_US.mo. You can use the PoEdit editor to compile the .po file.
