An existing User account can be upgraded to Administrator. This process is reversible - Once a User has been upgraded to Administrator, their account can be downgraded to User level using the same process.
From the Administration Block - click on Site Administration.
Click on Permissions.
Click on Site Administrators.
The "Manage Site Administrators" page has 2 panes - The left hand pane shows the Current Site Administrators and the right hand pane shows potential Users who could be added to the list of Administrators. Note that you may have to "search" for users to populate the right-hand pane.
Type the name (or part) into the right-hand pane search box and the names will filter according to the characters you enter.
When the User you are searching for is displayed, highlight the name and click on the "Add" button. The User should transfer into the left-hand pane (Current Site Administrators). A confirmation dialogue will be displayed where you have the option to Continue (save your changes) or Cancel (abandon your changes)
Removing an Administrator
Removing a User from the list of Admins is a simple reversal of the above procedure - Find the User in the left hand pane, highlight their name and click on "remove". A confirmation dialogue will appear and if you choose to "Continue" the account will be downgraded to a standard user.