VMS Help WRITE, Qualifiers, /UPDATE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Replaces the last record read with the record specified with the
    expression parameter. You must be able to read and write to a
    file to use the /UPDATE qualifier. Use the WRITE/UPDATE command
    only after a READ command. The WRITE/UPDATE command modifies the
    last record you have read.

    With sequential files, you must replace a record with another
    record of the same size when you use the WRITE/UPDATE command.