It's possible for your Android phone to update its OS after you've purchased it. Whether it can do so depends on your manufacturer's specifications about the model. Updates to the operating system are often automatic, but may also be initiated manually. This information applies to Android tablets, as well.
Manufacturer
Different cell phone manufacturers that use the Android operating system use different versions of the operating system on their phones. For example, HTC's Eris phone was still being sold with Android 1.5 when Android 2.1 was already available. Even within a manufacturer, since different models of phones come out all the time, each model may be running a different version of Android than its predecessors. Since manufacturers often tailor Android to fit their brand image, it's usually up to those manufacturers to issue Android OS updates for their range of phones.
Advice
Check with your phone's manufacturer to find out what updates, if any, are currently available for your particular Android phone. Be prepared to give the manufacturer your model information, as well as what cellphone carrier you use, because this may have bearing on what's available for your phone. If you're considering purchasing an Android phone, check with the manufacturer before purchase to find out what version of Android is installed on the phone, as well as whether updates are available. Then check to see whether the apps you want to install are compatible with that version of Android.
Considerations
Android OS updates are usually issued over-the-air, meaning your phone's manufacturer and your cellphone service provider often send an update to you automatically. That means, in most cases, that you don't actually have to do anything to start the update to your Android OS. It usually self-installs on your phone during whatever the network determines as its least busy time of day. However, in some instances, you may wish to install an update manually. When you contact your cellphone's manufacturer, a customer service representative should be able to walk you through the specific process of doing so, as it applies to your phone.
Future Devices
Like other operating systems and hardware combinations, your Android phone will eventually reach a point where the manufacturer no longer supports OS updates. This is often because your phone's hardware no longer has the power to support the latest evolution of the OS. At that point, you will need to continue running the latest compatible version of the OS, or you will need to purchase a new Android phone that can run more current versions of the OS.
Tags: your phone, Android phone, operating system, version Android, your Android, your phone manufacturer, Android updates