Enable Internet on your device with Android. Go to the settings section. Find the section where I will be talking about the system itself. Depending on the model this can be "On the system" "total system configuration" and the like.
Enable automatic updates system. If you don't want the OS updated itself, click "update" or "check for updates". If your model has a more recent version, it will automatically find them.
Be careful and use to download unlimited Internet or Wi-fi. The update weighs quite a lot and if the expense of the fix you can get in a lot of money.
Try to use Android Market if update via Internet does not work. In the market you can find either the update to your device, or a program that more details looking for updates, which for some reason could not find the device.
Go to the official website of the operating system and manually download the update if the above methods are unable to detect updates, but are you sure they should be. Go to the site, select your device and under "Download" view latest proposed version of your product. If indeed there is more recent, download it and just install in your device like a normal installer.
If you don't want to do the update yourself, you can entrust it to professionals. Take it to a computer or a mobile service on your device and within one business day you will change the firmware on newer, and at the same time will understand why not an automatic update.
In many ways, manual to update the OS depends on the model of the device that uses Android.