How to change Safari’s default search engine
STEP 1. Simply tap on Settings app from the home screen.
STEP 2. Now you have to tap on Safari.
STEP 3. Now tap on Search Engine.
STEP 4. Now simply choose the new Search Engine and then quit settings.
That’s it. You should note that until you change it again, your selection will remain the default Search Engine. In this way you can change Safari’s default Search Engine on your iPhone, iPod touch and iPad.