how can I remove yahoo from safari search?

On the search bar, click the button with a magnification glass:

Then it will show you some search engines, and one of them is chosen (in your case, Yahoo). You will be able to choose another search engine.

Sorry its Safari Preferences->Search->Search Engine.

Make sure its set to Google or whatever else you want to use that is not Yahoo.

Safari Menu->Preferences->General->Search Engine, make sure Yahoo is not selected.

Link->Change Search preferences in Safari on Mac - Apple Support