In the federal system, indictments can be superseded
but not amended. For information on that, see if you can get a copy of
my grand jury book from a law or other library, or check Wright &
Miller's multi-volume set on Federal Practice - check the section on
Rule 6 of the Federal Rules of Criminal Procedure, which governs the
return of indictments.
I assume indictments can be amended in some states,
but do not really know anything about that. You'd have to check state
law, if that is your interest. If you are checking state law, I'd start
with the state version of Rule 6, i.e., the state court rule or statute
that governs grand jury practice.