Hello and welcome to Android Forums. Hopefully I can help you with your problem.
1st, you need to find out if the 4.0 upgrade is available for your phone from your carrier. Just because the same model phone has the update, it may have a different carrier such as Sprint, Verizon, AT7T, Boost, etc. .
Contact your carrier and ask them if the 4.0 update is available for your phone. If it is they will walk you through the steps to get the update.
If you have any other questions regarding your device, I invite you to join Android Forums. We have many sections for specific devices which contain a wealth of information.