how to remove yahoo search in safari

You have adware/malware installed. Use  Malwarebytes Anti-Malware for Mac. It may be necessary to run more than one scan. After scanning a couple of times, restart the computer and test. 

Go to Safari Preferences/Extensions, uninstall any you don't recognize, and then turn all extensions off. Test. If okay, turn the extensions on one by one until you figure out what extension is causing the problem.

 Safari/Preferences/General - see what the Home Page is set to. If something you don't recognize, see if you can replace it with something you want.

 Safari menu/Clear History.

 System Preferences and remove any Profiles you see.

Please follow these steps to change your search engine, removing Yahoo Search from Safari. Please follow the relevant help article for your device.

Change Search preferences in Safari on Mac - Apple Support

Change the default web browser or email app on your iPhone, iPad, or iPod touch - Apple Support