Here's how you can quickly change the default search engine in Safari on iOS devices.
1. Open the Settings app.
2. Scroll down and tap the Safari icon.
3. Under the General section, tap on Search Engine.
4. You'll see a list of available search engines.
5. Click the search engine you want to use in Safari.
That's it. Now when you do a search in Safari on your iOS device, it will use your preferred choice instead of the default search engine.