I'm following the official guide to upgrade angular app to version 10 , currently while try to upgrade to CLI version 6 as per the update.angular.io
Make sure you are using Node 8 or later Update your Angular CLI locally, and migrate the configuration to the new angular.json format by running the following:
npm install @angular/cli@6
ng update @angular/cli@6
I've run the following code but still i could see .angular-cli.json file instead of angular.json file.
Below is current cli version updated to version 6 except for couple of warning related to peer dependency didnt get any error while updating CLI.

This did the trick !! @Andrew Allen but the second most voted answer worked in my case => you can automatically update your existing angular-cli.json file to angular.json file by using the below command provided that you are on v6.x.x or greater of angular cli command line tool.
In the above command 1.7.4 is the previous cli version you were using. The --migrate-only flag makes sure that it will only perform a migration but does not update the installed version.