What you are being notified about is not a free update, but an upgrade to Lightroom 6 or Lightroom CC if you are a subscriber to the Adobe creative cloud. Apparently that link hasn't worked for some people. You can click on the ignore button in the lower left-hand corner of that notification screen, and you won't see the notice anymore. If you want to upgrade then that upgrade needs to be purchased. And if you want to purchase the standalone upgrade it can be purchased from the following website:
You'll have to scroll to the bottom of the screen and click on the Buy button and indicate that you want an upgrade. After you indicate what version you are upgrading from the price will change.