How can I download the motion 5.0.2 update?

As far as I am aware there is no way to download an older version of Motion. Once the App store posted 5.0.3 5.0.2 was gone. That's why it's always a good idea to save your current version before updating. 5.0.3 seems a good update so why do you want the .2 version. All of your current and older Motion projects should be editable just fine in the new version.

Sure, Just go to the App Store (launch the app store application from your dock icon or applications directory) and select updates at the top of the page. It should show the motion update so just click on the update button. This assumes you have purchased an earlier version. If not then just go through the purchase process again assuming you have an account.