I have a blackberry 9650 and I was watching in updates that there is an available update to 5. something and I have a version 3 already installed so what is OS and what does it change in my phone if I update it
The OS is the phone's mobile operating system. Think of it like Windows, OS X or Linux on a computer. Upgrading the operating system can do lots of things: add more built-in apps, add more features or improve on old ones, it may use memory more efficiently and might help the device run faster.
The new version has to be compatible with your device, too. Some new versions might not run on older hardware.