This tip is for Windows SharePoint Services 3.0 and Microsoft Office SharePoint Server 2007.
First step test the update in a test farm. Second, test again. Third, make backups (Database and file system). Okay, now you can start with the upgrade.
Install the Windows SharePoint Services update/Service Pack on all servers in the farm. Install the Microsoft Office SharePoint Server update/Service Pack on all servers in the farm if applicable.
Run the SharePoint Products and Technologies Configuration Wizard on each server in the farm. A good start would be the server running the Central Administration service. Alternatively, you can run psconfig.exe -cmd upgrade –inplace b2b to accomplish this. Some upgrade errors can be fixed by running the psconfig.exe command with the psconfig.exe -cmd upgrade –inplace b2b -force switch also.