On Windows, both WinTD 5.00 and the earlier WinTD 4.20 allow you to download minor updates (newer builds of your software) at no charge. This is done using the "WinTD Updater" application in your WinTD desktop folder. WinTD Updater checks out web site and sees if there is a newer build. If there is, it will ask if you want to install it.
Note that this checks for a newer build of the software for which you have a license. For instance, minor updates of 4.2 will be a newer build of 4.2. If you have 4.2 and want to update to the newer 5.0, you need to purchase an update.
We do not currently have a similar system for the Macintosh. You can, however, re-install the software starting with downloading a new version from our web site. That will always get you the most recent build of the software.