Which version of Windows Phone do I have?
To see which Windows Phone operating system (OS) version is installed on your phone, do the following:
On Start, flick left to the Application list.
Press Settings > About > More info.The Software section indicates the general product name we use to describe the release (for example, "Windows Phone 7.8"). The OS version section indicates the specific build number of the release (for example, "7.10.8858.136").