Since these 1242AG access points connect to a WLC, they're running lightweight code. The lightweight APs actually get their software/IOS updates from the controller. In other words, you never need to upgrade the APs, just the controller. Once you upgrade a controller, the APs will automatically download their new code next time they connect to the controller.
To answer your question, the only real reason to upgrade is if there's a new feature or a bug-fix that you need in your production environment. If all is running fine, there's not really a reason to upgrade.
You can upgrade your controller code by going to the Commands tab in the web GUI. Click "Download File" on the left, then fill in the information on the right. You'll need a TFTP server that can transfer files over 32MB for this. I recommend Googling TFTPD32, it's a free quality TFTP server.