You cannot just install android, it needs to be built for the device.
If your manufacturer or carrier has made an Android 4 update for your phone, it will be available for download via the software updates menu (settings > about).
If you do not have an update available you may be able to do this by rooting the phone (voids warranty) and installing a custom ROM, if one exists for your phone. This is an android build which and independent developer has made for the device rather than the manufacturer. Some devices will have many ROMs, some none, depending on how many people were interested in doing this for that device.
Both root and ROMs are device-specific. As you've not said what phone you have, I can't say any more, except that you could see whether AF has a forum for your device, since that is where you will find more information.