The "amend" command:
Usage: fossil amend UUID OPTION ?OPTION ...?
Amend the tags on check-in UUID to change how it displays in the timeline.
--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
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"