Apart from changing the account type through computer Control Panel, you can change it through User Account Control as well. Detailed guides are:
1. Right click Computer and select Manage from Start menu
2. Click System Tools, then Local Users and Groups
3. Double click on Groups
4. Double click Administrators
5. Highlight the wanted account, then click Remove and OK
If you want to set a user account as the administrator, you can do the reversed actions. Or you can take a look at http://blog.doofix.com/how-to-make-a-user-account-the-administrator-of-the-windows/.
Here are a few useful guides. Hope this helps you.