Fed. R. Civ P. 15(a)
Amendments Before Trial.
(1) Amending as a Matter of Course.
A party may amend its pleading once as a matter of course within:
(A) 21 days after serving it, or
(B) if the pleading is one to which a responsive pleading is required,
21 days after service of a responsive pleading or 21 days after service
of a motion under Rule 12(b), (e), or (f), whichever is earlier.
(2) Other Amendments.
In all other cases, a party may amend its pleading only with the
opposing party's written consent or the court's leave. The court should
freely give leave when justice so requires.
(3) Time to Respond.
Unless the court orders otherwise, any required response to an amended
pleading must be made within the time remaining to respond to the
original pleading or within 14 days after service of the amended
pleading, whichever is later.
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(c) Relation Back of Amendments.
(1) When an Amendment Relates Back.
An amendment to a pleading relates back to the date of the original
pleading when:
(A) the law that provides the applicable statute of limitations allows
relation back;
(B) the amendment asserts a claim or defense that arose out of the
conduct, transaction, or occurrence set out — or attempted to be set
out — in the original pleading; or
(C) the amendment changes the party or the naming of the
party against whom a claim is asserted, if Rule 15(c)(1)(B) is satisfied and
if, within the period provided by Rule 4(m) for serving the summons and
complaint, the party to be brought in by amendment:
(i) received such notice of the action that it will not be
prejudiced in defending on the merits; and
(ii) knew or should have known that the action would have
been brought against it, but for a mistake concerning the proper party's
identity.
(d) Supplemental Pleadings.
On motion and reasonable notice, the court may, on just terms, permit a
party to serve a supplemental pleading setting out any transaction,
occurrence, or event that happened after the date of the pleading to be
supplemented. The court may permit supplementation even though the
original pleading is defective in stating a claim or defense. The court
may order that the opposing party plead to the supplemental pleading
within a specified time.