change default search engine

If you mean the built in search window at the top right of Safari, yes... just click the arrow to the left to see the choices in my screen cap.

 

I don't know of a way to change that without using a plugin (like Glims mentioned). Plugins can cause instability with Safari sometimes. That said, I have not tried Glims.

There is a Safari extension called Popsearch that will give you a popup search box for any search site you want to use. I have not tried it either, but I use a couple of the same devs other extensions and they work well.