Where can I change name of "search" button?

I just looked at the theme and it looks like it doesn’t have a search form but instead uses the default WP search form.

if you look at this page on the Codex – https://codex.www.ads-software.com/Function_Reference/get_search_form
the third examples shows you how you can override the search with your own filter.
Maybe try that and change the value of the search button in there?