The DotNetNuke Wiki has some good info on the upgrade procedure and in particular a suggested version upgrade path:
http://www.dotnetnuke.com/Resources/W...
You will also want to click on the links contained on the related Wiki pages and in particular take a look at Will Strohl's blog.
Older versions of DotNetNuke are found on the DotNetNuke framework's CodePlex project site:
http://dotnetnuke.codeplex.com/releases. There are also links to those same download pages in the Wiki's list of suggested upgrade paths. I usually recommend upgrading to 4.6.2, 4.9.5, 5.4.4 and finally to 5.6.2 (or 5.6.3 which will soon be released to fix several critical issues in 5.6.2).
A couple of other points to keep in mind:
1. The upgrade to 4.6.2 will require manual modification of the version 4.6.2 package's web.config file to include connection strings, machine keys, object qualifier, etc. from your 4.5.4 site's web.config. All other upgrades should use the upgrade packages which do not include a web.config file as modifications to the existing site's web.config will be done automatically.
2. DotNetNuke v 5.2.0 and later require ASP.Net v 3.5 SP 1 and MS SQL Server 2005 or 2008. DNN v 5.4.4 and later can also use ASP.Net 4.0 if desired.
3. Check the various Forge module forums here at DotNetNuke.com as well as their project pages on CodePlex to learn of any version incompatibilities with "core" modules. For example, the XML and Chat modules will need to be uninstalled or upgraded prior to the upgrade to DNN 5.4.4. Likewise check with the developers or venders for any third-party modules you may have installed.
4. BACKUP - BACKUP - BACKUP both the the file system and database before each version upgrade and retain each backup set should you need to roll back one or more versions due to upgrade errors. Did I mention BACKUP - BACKUP - BACKUP ??!!