The "amend" command:

Usage: fossil amend UUID OPTION ?OPTION ...?

Amend the tags on check-in UUID to change how it displays in the timeline.

Options:

   --author USER           Make USER the author for check-in
   -m|--comment COMMENT    Make COMMENT the check-in comment
   -M|--message-file FILE  Read the amended comment from FILE
   -e|--edit-comment       Launch editor to revise comment
   --date DATETIME         Make DATETIME the check-in time
   --bgcolor COLOR         Apply COLOR to this check-in
   --branchcolor COLOR     Apply and propagate COLOR to the branch
   --tag TAG               Add new TAG to this check-in
   --cancel TAG            Cancel TAG from this check-in
   --branch NAME           Make this check-in the start of branch NAME
   --hide                  Hide branch starting from this check-in
   --close                 Mark this "leaf" as closed
   -n|--dry-run            Print control artifact, but make no changes
   --date-override DATETIME  Set the change time on the control artifact
   --user-override USER      Set the user name on the control artifact

DATETIME may be "now" or "YYYY-MM-DDTHH:MM:SS.SSS". If in
year-month-day form, it may be truncated, the "T" may be replaced by
a space, and it may also name a timezone offset from UTC as "-HH:MM"
(westward) or "+HH:MM" (eastward). Either no timezone suffix or "Z"
means UTC.