You need to be an administrator to install updates. It sounds like you are attempting to update from a Standard User account. In which case you should had been asked to have an administrator authorize the update. In which case you will be given a dialog box where you enter the Administrator Accounts' User Name and Password.
The Administrator Account is the first account you created when you initially set up your computer. (Though you can latter change this account into a Standard User account.) Many people will set up a second, Standard account for their everyday use. Thus providing an additional gate that high jackers must pass through in order to install malware.
Check in System Preferences>Accounts to see if you are operating as an administrator or a standard user. And to see who the computers administrator is if you are a standard user.
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The Administrator Account is the first account you created when you initially set up your computer. (Though you can latter change this account into a Standard User account.) Many people will set up a second, Standard account for their everyday use. Thus providing an additional gate that high jackers must pass through in order to install malware.
Check in System Preferences>Accounts to see if you are operating as an administrator or a standard user. And to see who the computers administrator is if you are a standard user.