Whomever did the Template did not do you any favours. Instead of a proper setup using the override system and creating a custom Template for you, they simply used the files in the Classic dir. This means when you do an update, all your customization will be lost. You should have someone properly code your Template for you ASAP.
With that said, you could try to replacing the Create Account page which would be found in one of two places.
First, see if you have the file here:
store/includes/templates/template_default/classic/tpl_modules_create_account.php
else it will be here:
store/includes/templates/template_default/templates/tpl_modules_create_account.php
You will probably need to replace the file with one from the original download of Zen Cart. You can download the original files if you do not have what you need.
The learning is in the doing.
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