update mail

I am running an iMac Mac OSX 10.6.8 with the mail version 4.5. When I click on the mail icon, I receive the following message. You can't use this version of Mail with this version of Mac OSX. You have Mail version 4.5 (1084/1085). It can't be used on Mac OSX Version 10.6.8 (build 10K549). For more information, click the Help button.

How do I update the mail application in order to use it?

Thank you.