how do I remove yahoo search engine from my macbook?

Malwarebytes is the one exception to this rule. It makes very few false detections and has proven to be very effective at removing malware. See this thread, where apple has recommended it as a solution: https://discussions.apple.com/thread/252716782.

A unwanted redirect to Yahoo?

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. If necessary, follow the manual instructions.  Do not download any other programs that are listed in the articles.  

See if the below helps. I would scroll down to the below or you might fall asleep.

Step 1. Eliminate Yahoo Redirect from Windows.    

Step 2. Delete Yahoo Redirect from Mac OS      

Step 3. Uninstall Yahoo Redirect from Internet Explorer      

Step 4. Remove Yahoo Redirect from Microsoft Edge.     

Step 5. Erase Yahoo Redirect from Mozilla Firefox.   

Step 6. Get rid of Yahoo Redirect from Google Chrome.    

Step 7. Eliminate Yahoo Redirect from Safari.   

https://www.2-spyware.com/remove-yahoo-redirect-virus.html.    

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.

You are welcome. As stated above, Malwarebytes is an exception.