Upgrading is pretty much the same as initial installation. There is documentation here:
As long as your database connection string and custom settings are in user.config, you basically deploy the new files on top of the existing ones and then visit /Setup/Default.aspx to complete the upgrade.
You should ideally backup your database first and the current files, but at minimum, backup your Web.config file in case there are any custom sttigns there that you need to add into the new Web.config file.
Hope it helps,