WordNet records no synonym and no antonym for amendment. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the act of amending or correcting
a statement that is added to or revises or improves a proposal or document (a bill or constitution etc.)
Synonyms, antonyms and the broader and narrower relations for amendment come from Princeton University’s WordNet. Full sources and licences.