The SQL Server 2008 Service Pack that can update any edition can be found here. The Express 2008 SP1 release can be found here. This service pack makes available Slipstream and SP uninstall.
The Slipstream functionality in Service Pack 1
There are 2 different procedures that allows you to patch the original SQL Server 2008 product
Procedure 1: Basic Slipstream steps:
These steps are better for a single installation as the steps are simpler since you don't need to copy the original media locally, all SP architecture types do not need to be download and there is less of a chance to make a mistake.
See http://blogs.msdn.com/petersad/archive/2009/03/02/sql-server-2008-basic-slipstream-steps.aspx for details.
Procedure 2: Creating a Merge (slipstream) drop:
These steps will guide you in creating a drop from where can install SQL Server 2008 SP1 in a single step.