Upgrade from 2.3.1

There isn’t a WP 2.4.

So you need to do a full file & database backup, then disable your plugins & upgrade from

• 2.3.1 to 2.5
• 2.5 to 2.8.5
• 2.8.5 to 3.0.1

Then enable your plugins one-by-one to check that they still work.

You’ll find the archived WordPress versions at: http://wordpress.org/download/release-archive/