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Gallwch. Gallwch ddiwygio Ffurflen Dreth rydych eisoes wedi'i chyflwyno i Gyllid a Thollau EM (HMRC). Gallwch anfon y diwygiadau ar-lein neu ar bapur.
Mae arweiniad ar sut i ddiwygio Ffurflen Dreth ar gael ar y dudalen we ar gyfer Treth Gorfforaeth: sefydlu a rheoli eich cyfrif ar-lein.
