How do I remove yahoo search from safari?

Look at this thread https://discussions.apple.com/thread/7691255

Force Quit Safari ( command + option + esc keys).Then restart Safari holding the Shift key. 

If you still have problems Empty Caches (Safari menu > Preferences > Privacy > Remove all website data. (This will also remove history if you do not want to remove History open Safari Preferences > Advanced and check mark “Show Develop Menu” then choose “Empty Caches” from Develop Menu ).

Then go to Safari menu > Preferences > Extensions and check there. If there is an extension delete it.

Also check in System Preferences > Profiles (if you have that pane). If there's anything in there you don't recognize delete it.

If the problem persists then download and run Malwarebytes. Malwarebytes was developed by one of our own colleagues here in ASC and is about the most proven anti-malware software for Mac.

Restart and reset your default bowser again.