https://www.law.cornell.edu/uscode/text/18/700
(a)
(1) Whoever knowingly mutilates, defaces, physically defiles, burns, maintains on the floor or ground, or tramples upon any flag of the United States shall be fined under this title or imprisoned for not more than one year, or both.
(2) This subsection does not prohibit any conduct consisting of the disposal of a flag when it has become worn or soiled.