Since Google Chrome & Firefox use Google as the default search engine, & Internet Explorer uses Bing as the default search engine. I don't think so there's a need to change default search engine for Google Chrome & Mozilla Firefox users. And if Internet Explorer users want to change, or even Google or Firefox users need/want to change, it's pretty simple. It just takes less than 30 seconds to change your default search provider.
There are several reasons, though, some of us just keep installing different add-ons/softwares, etc. & get their system infected by a virus. So it automatically gets changed by adware/malware. So if you're one of them, I'd suggest you scan your system by Avast or any other "good" antivirus & then use this method to change your default search engine.
Google Chrome
Go to settings.
Select Google, or your preferred search provider.
Mozilla Firefox
Click on the search button & change search settings
Under default search engine you can select your preferred search engine. If you want to add more search engines, click on add more search engines & it a new window will open.
Click on add to Firefox & it will be added in Firefox without any hassle.
Internet Explorer
Click on settings > manage add-ons
Restart your browser, go back to search providers & you see Google has been added. Right click on it & click set as default
