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
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(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.