Near the top right side of your screen, there’s the “hamburger” button (looks like three lines stacked on top of each other). Click on it and go to settings.
If you scroll down the page, you’ll find a heading called search engine. There, you’ll find an option to let you change your search engine in the address bar.
If it’s the Android version, as Matches supposes, push on the three dots on the top right side of the screen and scroll down to Settings. The very first option that comes up there lets you choose your search engine.
Personally, I’m fond of DuckDuckGo or Start Page. Both of these search engines don’t track you. Qwant is also popular. Searx is another possibility. You’ll also find some other, popular options.
I hope that helps!