Hi Mildo,
Sounds like your theme’s css is transforming the button text to lowercase. You need to add some custom css to override this behaviour:
.bop-nav-search input[type="submit"]{
text-transform: none !important;
}
If your theme is not a third party one (i.e. your own to some extent), just go ahead and add it to your style.css. However, if it is a third party one, you’ll have to do something else to avoid losing your changes when the theme updates. Your theme may have its own allowances for custom css in the admin area (typically in Appearance > Customise), use that if its available. Otherwise, you’ll need a custom css plugin and add it in there instead. Alternatively, you could create a child theme of your current one, though this is the most complicated route and not really worth it for such a small change.
Cheers,
Joe