Did you mean OS X Safari instead?
Changing Your Search Engine to DuckDuckGo
Open the Settings app
Tap "Search Engine"
However, please know that if you just use the search engine, we cannot offer our web tracking protections when you visit other web sites. To get that protection, you need to use our extension or browser. Our free web browser is available for Mac, Windows, iOS, and Android . It comes with seamless privacy protection built-in, including private search, tracker blocking, increased encryption, email protection, and more.
Prior versions on GitHub.