First off you have to be on an administrator account to do this.
There are several ways to do this.
Probably the easiest way is to go into control panel, select User Accounts. Click on Change an account, then select the account you want to change. Click Change the account type, select Computer Administrator, then click Change Account Type.
The one I use, is go to Start -> run -> type control userpasswords2
Click on the account you want to edit, then go to properties. Click on the group membership account. Click on Other, and in the dropdown select Administrators
A second way to do this is by right clicking on my computer, go to manage. Under the Local Users and Groups Tab, click on users. Right click the account you want to edit. Click on the Member Of Tab. In the "Enter the object names to select" box type Administrators. Then select the old group memebership and click on remove.
