The /usr/src/UPDATING is updated as part of the source with CVSup. It is very important to take a few minutes to read this document before proceeding.
The UPDATING file dates back to before 4.0-RELEASE, and contains notes of various modifications to the source that may cause problems to people updating their systems, as well as various other notes useful in that situation.
The /usr/src/UPDATING for any version can be accessed at http://www.freebsd.org/cgi/cvsweb.cgi/src/UPDATING. Be sure to select the correct version or you may become confused.
You should also read the appropriate mailing list for a week or two prior to updating your system. The /usr/src/UPDATING can lag behind a little or there may be new items being discussed that could affect the success of your updating.