There are dozens of articles and white papers regarding migrating SBS version 20xx to version 20xx but many people seem to have difficulty locating these. The following is a collection of some of the more popular options and methods.
Firstly there is no upgrade option, and if you have never done a migration I strongly recommend carefully reviewing documentation and try a migration in a test lab first as it is a lengthy procedure due to all the components included in an SBS environment. You might want to considering hiring someone experienced with doing so, or perhaps buy a Migration “Kit” from swingmigration.com SwingMigration.com specialize in migrations, and in particular SBS. They provide detailed documentation for you specific migration scenario, some basic tools, 90 days support for the migration, and a method that allows you to revert back to your original configuration at any point.
If you want to go it on your own, or just read up on the topic, thee links may be of some help.
SBS 2003 to SBS 2003
SBS 2003 to SBS 2008
SBS 2003 to SBS 2011
SBS 2003 to SBS 2011 Essentials
SBS 2003 to Server 2008 R2 and Exchange
Server 2003 standard with Exchange to SBS 2008
SBS 2008 to SBS 2011
SBS 2011 to SBS 2011