Changing applicaiton pool account

A mistake was made during setup and an actual user account was used for the application pool acount. If I simply go to SharePoint application pool and change the identity to Network Service the SharePoint web site show "Cannot connect to the configuration database." How can it be changed now Which account should be used for administration application pool

Your help is appreciated.

SharePoint Products and Technologies2

You should run the application pool with a user account - create a system user (for example sys-SharePointAppPool) and use it for the application pool.

But since that account needs access to the databases, you should also go into SQL and give it security permissions to the database manually.