If your search engine is defaulting to Yahoo,
it could be due to malware.
Be careful not to click the pop-ups like this
when surfing the internet.
And you can try following these steps to fix it.
To reset Chrome browser settings,
click the three dots at the top right of the browser,
then click "Settings."
Click "Advanced" to expand advanced settings,
then click "Reset settings."
Finally, click "Restore settings
to their original defaults,"
then click "Reset settings" to confirm.
Click "Safari" on the menu bar, then click "Preferences."
Go to the extensions tab
and uninstall any suspicious extensions.
Then go to the search tab.
Pick the search engine you want to use,
for example, Google.
Finally, go to the general tab.
Change homepage to the one you like,
for example, www.google.com.
To reset Firefox browser settings,
click "Help" on the menu bar,
then click "More Troubleshooting Information."
Finally, click "Refresh Firefox."
Then click "Refresh Firefox" to confirm.