Version 1.7 download (see install instructions)
This is the complete install, for upgrades, see below.
Go to your blog administrator and you should see a button to let you run the automatic upgrade.
If for some reason you can't run the automatic updater and you are in version 1.3.1, you can download the update, unzip it and follow the instructions in the readme.txt file.
If you installed version 1.4.1, you will need to make sure your role has the "Manage system" permission. If you don't, you will not be able to download the update. Go to Users > Roles, and click on your role. The select the "Manage System" checkbox and click Submit. The logout and log back in.