My search engine is Yahoo, how can I change it to Google?
You set this in Safari* Preferences - you don't need an extension to do this.



In Safari, go to the Search tab in Safari → Preferences (or press ⌘ Command,)

Select the Search Tab, and pick your Search engine.  I personally use Bing (by the way, Yahoo uses Bing to power it's search engine since 2009), but you can choose the engine you prefer.


* Safari 11.1.2 was used at the time of submitting this answer