Currently, MediaWiki is at release 1.15.1. If you recently installed the software, you may not be able to upgrade because a newer version may not be available yet. If you are using an installer such as Fantastico, or one provided by your hosting company, then when you log in you should be alerted when a new release is available for MediaWiki.
When the time comes that you need to upgrade your MediaWiki installation, these are the steps you need to follow. Many people simply click on the upgrade button from their hosting control panel. Failure to upgrade the right way can really cause some serious damage to your site.
1. Check the requirements. Make sure that you are running the correct version of PHP and MySQL, or PostgreSQL. If you are not, you will need to upgrade these first.
2. Read the release notes. These are available to you on the MediaWiki site corresponding to the release you are installing. If everything checks out, then proceed.
3. Back up your...