Platinum versions come with 2 years of access to the updates website allowing any new features and bug fixes to be downloaded for free. One that has expired, you will need to purchase additional years if you want to always have the latest version. There are no longer any major update versions you have to buy, it is all included in the update access so it makes sense to keep your access alive.
You can now see the available purchase options for extending the update access by going to the help menu of the program and selecting "Purchase add-ons" folowed by "see options on Internet". This will display only items that relate to your edition of the software.
Extra years can be purchased at any time and it is suggested you do so before your current expiry date to prevent having to wait for access to be extended. When renewing already expired access, note that the time period purchased runs from the previous expiry date and not the date you renew.