I updated to 2.0.14, with no incident. Then, I was told that there was an update to 2.6.1. In each case, I had to download the installer from the Mozilla site. The new Seamonkey was installed in a different folder than the default SM, and I used a different profile. But, updating to 2.6.1 caused the disappearance of the contents of the bookmarks folder. The solution unexpected, but easy. From the Bookmarks|Manage Bookmarks|Tools|Import, I imported the bookmarks-backup.html file. Weird, because it was in the same profile folder. Everything seems in order, now.
And, checking for updates, tells me that 2.8 is the new update.
The moral of the story is that it is possible to update, with no problem, from 2.0.11 to 2.0.14, and from 2.6.1 to 2.8, but not from an earlier version to 2.6.1, or 2.8.
UserAgent: Mozilla/5.0 (Windows NT 5.1; rv:9.0.1) Gecko/20111221 Firefox/9.0.1 SeaMonkey/2.6.1