If you have most recent version of wordpress (wordpress 2.6 or latest) you can automatically update WordPress by clicking the WordPress is available! Please update now. link found within your menu bar. Once you click the link Please update now, you will be taken to a page where you want to update wordpress automatically or do it manually.
Click Upgrade Automatically Button, to upgrade you wordpress installation easily to the latest stable release of WordPress provided by wordpress.org.
Image: WordPress auto upgrade options
Automatic upgrade upgrades your wordpress installation by doing the following steps.
Downloading update from http://wordpress.org/wordpress-<version>.zip
Unpacking the core update
Verifying the unpacked files
Installing the latest version
WordPress upgraded successfully
After you see WordPress upgraded successfully message, you have upgraded to the latest stable version of wordpress ( state-of-the-art publishing platform) automatically without any hassel.
Note: Make sure to backup you themes and database before upgrade. If you use wordpress default theme and made modification to it. Your changes will be lost while upgrading (I ran accross this WordPress Upgrade Issue for one of my blog ), so make a backup of your default theme.