upgrading from 2.1 to 2.3
Sure, have a look at the Craft site for details about downloading a specifc build of Craft:

http://buildwithcraft.com/help/download-previous-versions

So if you want to download 2.2.2579, the URL will be:

http://download.buildwithcraft.com/craft/2.2/2.2.2579/Craft-2.2.2579.zip