Civil Procedure I

Amend

As a matter of right, a complaint or answer can be amended one time within the greater of 21 days or 21 days after the first Rule 12 motion or responsive pleading. Rule 15(a).

Otherwise, amendments should be "freely give[n] when justice so requires". Rule 15(a)(2). The court must weigh the prejudice to the defendant against the need by the plaintiff.