Just so we can discuss this with clear terminology. there is no such word as upgradation, and a service pack isn't an upgrade. It's an application of a patch to the server.

What level are you going from to what level on the server? Is this an RTM, base version of SQL Server 2005? What service pack are you applying?

The basic steps are listed above from rambill4. Download, read the instructions, and it's generally an install like any other sofware. Make sure you have backups, and time to reinstall SQL Server if there are issues.

You ought to have tested this on a test server, and WITH YOUR application to be sure that a change won't break anything.