Security is always a concern. It's important to update your core Joomla! installation and your added extensions regularly. A popular tool for updating the core is
Update Manager for Joomla!. It installs as a regular extension, and then provides a one-click method for updating Joomla!. Caution - if you have modified core Joomla! files, you may overwrite them during the update. Which is one reason you should not be modifying core files, unless all else fails. Stick with template overrides whenever possible.
Some individual extensions require you to uninstall and reinstall, and others can be installed on top of the current version. Especially if you have more than one site, it helps to keep a spreadsheet with all of your installed extensions, their version numbers, url where you downloaded the extension, and install date. There is no one click method for keeping your extensions updated - you need to manually check them on a regular basis.
