How to turn an administrator user account into a standard one?
Search for user accounts in System settings , there you will get window like this
Now select the account you want to change to Standard One, Unlock it from the Upper right button which will ask for password.
Now selecting the Account type Button ( refer image) , you will get option as Standard. It will again ask for password . Now it is done.
For Doing it through Command Line Refer the link provided by adempewolff in the above comment.