You can find an upgrade pat on the wiki

http://www.dnnsoftware.com/wiki/page/suggested_upgrade_path

Forge projects : Open Url Rewriter - Open Content - Open Form

Hey Leffan,

The first thing you would need to do is work out what modules are used by the older 3.0 - and more importantly - are there any custom modules - if there are custom modules then it can be a more complex process.  Check especially to see if the SKIN has any custom menu objects - these are likely sources of issues.

Your safest bet after that - make a backup of the full existing site and database - and install that on a local development computer - and get it running properly.  

After that - you can start running the various update steps on the local version - as outlined in http://www.dnnsoftware.com/wiki/page/suggested_upgrade_path

After each step - test the site and make sure it all looks ok - IF there were any custom modules - make sure to test those pages closely - also again check menus and any custom skin elements.

As a side note - I would maybe consider doing an update to a new skin - you can get a very quick site look and feel refresh from a skin update - and pick up features like mobile/responsive layouts - while did not  exist back in 3.0

Westa