Go to this link and click on the download link.

http://www.mozilla.org/en-US/thunderbird

It's no use downloading the entire program from scratch. I found a way around that by updating through MacKeeper. It has a Updates Manager tab that tells you what needs updating and actually does the update for you. Not good Mozilla that another program updates your software better than you.

Why the objection to "downloading the whole program from scratch"?

How do you know this isn't what your Updates Manager does anyway?

If there is no update path using a patch from one version to another, then you have to download the installer for the newer version. In this case there would be no "upgrade" path; it would be a re-install.

It is an ongoing irritation with Thunderbird that occasionally there is a version change which the old version doesn't "see" and so you have to download the new version. Possibly this is deliberate to force a complete reinstallation rather than apply a patch, perhaps because there has been some fundamental change which is beyond the capability of a patch. In the past, some updates have changed the profile structure, and the new version of Thunderbird "knows" how to update the profile too.

But reinstalling "from scratch" is no big deal; your data is stored separately in Thunderbird's profile, which persists from one install to another. Indeed, you can uninstall Thunderbird, install a newer version at some later time, and it'll pick up your old profile and work as if nothing had happened.

Thanks Zenos. I understand what you are saying. In any case, I was able to update using the update functionality within MacKeeper, so all is well.