I think UPGRADE.txt should be completed with instructions for minor updates (like 6.x -> 6.y).
It seems many Drupal users are having trouble with those frequent updates (for bugs and security), for them a very time consuming task due to the confusion between minor updates (6.x -> 6.y) and major upgrades (5.x -> 6.y).
The current UPGRADE.txt is in fact a procedure for major upgrades, with steps such as: switch to a core theme, disable all contributed modules, ensure that the versions of all modules match the new Drupal version, re-enable all modules, etc. For minor updates this is an unnecessarily long procedure.
See what some users are saying, for example:
So you don't disable all modules? You are skipping a lot of steps in UPGRADE.txt, such as disabling all custom and contributed modules. I upgraded 6.6 -> 6.8 recently, and this was the most painful part... especially since disabling all modules requires multiple passes due to dependency issues, and you need to keep track of exactly which modules you need to re-enable after (and my site uses a LOT of modules).
(From the thread: Is a different approach in upgrading possible?)
I have exactly the same problem as Anil, it is so complicated and time consuming keeping Drupal up to date. In WordPress you simply click the update button and it gets on with it. Really, the hassle of doing manual updates is a deal breaker for me. I'm more than likely moving to WordPress.
(From the thread: Updating a minor version requires so much grief?)
In other more complete instructions, like Acquia Drupal's "Getting Started Guide" (from Drupal founder's company), or O'Reilly's book "Using Drupal" (from the Lullabot consulting team), there is not disabling of modules for minor updates (and it's only mentioned by Acquia as an optional step for major upgrades). A new version of UPGRADE.txt should reflect this.