How can i change ,Mozilla to Google for my default browser?

I don't get it. You have a Chromebook and want to install Mozilla as your default browser? No can do. Chrome is the OS and the thing that ties everything in that computer.

You mean you want your Mozilla (Firefox) browser to use Google as your default search engine? Easy, it's in Settings.

First off there are a couple of things you need to learn.
Mozilla and Google are NOT Browsers.
Firefox is the Browser that Mozilla provides and CHROME is the Browser that Google provides.
First you need to Download and Install CHROME.
Then in Windows 10 you click on START > Settings. In Settings you click on System.
In System you Click on Default Apps. Go down the list and find WEB Browser and set it to Chrome.

I'm going to assume you are on Windows 10.
Click the windows button on the bottom left the screen, click on the settings icon.
Click on System, select the Default Apps tab.
Scroll down until you see "Web Browser."
Select the browser you like.

You may have to download Google or Chrome first.

Thanks to all for suggest me.

Check the settings for Chrome. It would show the option as well as in the Control Panel/System Preferences.

Use your Internet settings in the tool menu.