remove yahoo search engine from my iMac

The free Malwarebytes which may take care of it...

https://www.malwarebytes.com/mac/

To remove a configuration profile in macOS:

1. From the Apple menu, select System Preferences....
2. From the View menu in System Preferences, select Profiles.Note:
3. Profiles won't be visible until you have at least one profile installed.
4. Select the profile you want to remove, and then press the - (minus) button. Click Remove to remove the profile.

softwater

User level: Level 5 (5,557 points)

Feb 16, 2020 3:04 AM in response to sukwaria

If your browser prefs are locked down, you'll probably find the reason in System Preferences > Profiles. 

Remove all profiles you don't recognize.

Open Safari preferences, click on the Search button in the toolbar, then change the search engine.

How'd it go?