Change text on search form from zip code to postcode

Hi

I'm working on building a directory for a client, and need to change the text on the form (form label basically) from saying 'search users by zip code' to something like 'search by postcode' or 'search users by postcode'

The site is here- https://goo.gl/sWuav2 and you can see the form i'm referring to on the home page, but its also on other pages (like listing pages) so ideally i want to change the term across the site.

I've tried to search the term in the database as well but can't find it in there for some reason.

I've granted support access on the site too.

Thanks
Farzan