Thank you for your question.
What is theme you are using.
You can find the code in theme.
You can try follow this path:
Themes => edit code => template => password.liquid
normally in that file will contain some sections, so I think you can find the code in section 'password-content', that is follow the theme debut or some shopify theme
I'd be happy to help. I just had a couple of quick questions:
1. What is the name of the theme you are using?
2. Are you using an app to power this password page? If so which one.