If you want to keep WordPress blog secure, you have to upgrade to the latest release. Always check for the security release by wordpress team and upgrade to the latest version. Before upgrading check for your current version of wordpress to make sure which one you upgraded.
If you have most recent version of wordpress (wordpress 2.6 or latest) you can automatically update WordPress using the ink found within your menu bar.
If you want to check check the current version of wordpress daskboard, within dashboard you will find what is the current version installed along with post count etc.
Dashboard will display like this
You are using WordPress 2.8.2.