Answer by eddieringle · Sep 12, 2010 at 01:31 AM
Unless you've rooted your phone, you cannot manually upgrade your device to a later version than the phone manufacturer and carrier allows. If an official update has been released and you still have not gotten it, you can upgrade through the Android Settings app (I believe it will be under "About phone").
If your device is rooted, you can just flash updates yourself. Check out XDA-Developers for more information on that.
Answer by DJ Scooby Doo · May 30, 2012 at 01:48 PM
If the carrier hasn't pushed an update, you can usually flash it yourself by following instructions via XDA-Developers like @eddieringle said, or if it's supported by Cyanogenmod, you can flash it there and usually it has a higher firmware than what the carrier bundles.