Download the installer from; Download Firefox For All languages And Systems {web link}

Be sure to choose the right installer for your system and location.

When running the installer, you should be asked if you want Firefox to be your default.

If Firefox is already on your system; Start the browser. Type about:preferences#general<enter> in the address bar. Turn on Make Firefox Default Browser. You can also turn on; Always Check Firefox Default Browser.

ButcherBob said

My browser is google now how can I change it to firefox

Firefox itself is a web browser as by Google I am guessing you are seeing it as the default Search Engine. Google is the default on some locales of Firefox since Fx 57.0 and later https://blog.mozilla.org/blog/2017/11/14/firefox-features-google-as-default-search-provider-in-the-u-s-canada-hong-kong-and-taiwan/

You can change the default search engine. https://support.mozilla.org/en-US/kb/add-or-remove-search-engine-firefox https://support.mozilla.org/en-US/kb/change-your-default-search-settings-firefox