The "undo" command:

Usage: fossil undo ?OPTIONS? ?FILENAME...?
   or: fossil redo ?OPTIONS? ?FILENAME...?

Undo the changes to the working checkout caused by the most recent
of the following operations:

   (1) fossil update             (5) fossil stash apply
   (2) fossil merge              (6) fossil stash drop
   (3) fossil revert             (7) fossil stash goto
   (4) fossil stash pop

If FILENAME is specified then restore the content of the named
file(s) but otherwise leave the update or merge or revert in effect. 
The redo command undoes the effect of the most recent undo.

If the --explain option is present, no changes are made and instead
the undo or redo command explains what actions the undo or redo would
have done had the --explain been omitted.

A single level of undo/redo is supported.  The undo/redo stack
is cleared by the commit and checkout commands.

Options:
  --explain    do not make changes but show what would be done

See also: commit, status