From time to time, you will find Drupal updates with security and other fixes. You will need to keep your site up-to-date with the latest available release. This can be quite time-consuming in general, but Drupal makes it relatively easy.

When you download the latest release, you will typically find an "UPGRADE.txt" file that gives you detailed information. Reading this file and following the instructions is quite important, as you could crash your installation quite easily by missing some important steps.

This is what I have done to upgrade Drupal in my sites, but I try and simplify the process by following these steps.

1. Login and administrator and revert to default theme as recommended. Then, put the site in offline mode.
2. Move your entire Drupal installation directory to another. For example, rename it from drupal to drupalPrev. This way you are backing up the entire set-up.
3. Copy your new Drupal files to the destination directory.
4. From your copy of the old installation, copy things like settings.php, files, modules, etc, as documented in the upgrade instructions.
5. Run the upgrade.php as instructed.
6. Check for status messages and upgrade any modules as recommended.
7. Put the site back in online mode.

Basically, I follow the same procedure as suggested in UPGRADE.txt, but I make the job easier by renaming the entire old set-up directory. This way, you don't need to spend time figuring out what to backup and what to delete.

Once your new installation has been tested well, you can clean up the renamed version or back it up as you prefer.