It is always advisable to upgrade your wordpress blog to the latest version. However, I have seen that a lot of webmasters have not upgraded their wordpress blogs and are still using wordpress versions as old as 1.5, 2.0 or 2.5. Most of their concerns are that they do not want to go through the process of backing up their files, databases etc.
But many webmasters do not realize that the main problem of using an old version is that there might be various security issues with the older version and so your blog is more vulnerable to hack attempts etc. WordPress officially recommends to upgrade to the latest versions and it is also highly advisable to upgrade from a security viewpoint. Recent versions of wordpress have also made upgrading easier by allowing automatic upgrade instead of the manual upgrades.
If you run older versions of wordpress you normally have to go through the manual upgrade process which is quite tedious. But the best part is that there is a plugin to automatically upgrade your wordpress blog from as old as version 1.5 to the latest version. The plugin is called wordpress automatic upgrade (WPAU) and can be downloaded from the following link.
WordPress automatic upgrade (WPAU) upgrades your wordpress installation by doing the following steps.
1. Backs up the files and makes available a link to download it.
2. Backs up the database and makes available a link to download it.
3. Downloads the latest files from http://wordpress.org/latest.zip and unzips it.
4. Puts the site in maintenance mode.
5. De-activates all active plugins and remembers it.
6. Upgrades wordpress files.
7. Gives you a link that will open in a new window to upgrade installation.
8. Re-activates the plugins.
So, add this plugin in your plugin folder and activate you like you do with any other plugins.
Then you will see the instructions on your screen when you go to Manage-> WordPress Automatic Upgrade and simply follow the instructions.
You might be asked for your FTP details if the files are not writable so that the plugin can automatically manage the file permissions. The plugin will do all the work for you as mentioned above including backing up the database and files, deactivating the plugins, upgrading and activating the plugins again. You will also be provided link to download the backed up database and files and make sure that you download those so that you can revert back in case there is any problem. The database upgrade link would also be provided while you go ahead with the automatic upgrade and please make sure you upgrade the database too.
So, upgrade all your wordpress installations to the latest versions if you have not already done it.
NOTE: It is advisable to keep personal backup of your website if possible so that you can roll back to the previous version if any problem occurs in upgrade. Also check the plugin page for FAQ and other details.
The instructions for manually upgrading your wordpress blog can be found here just in case you face any problem with automatic upgrade.