My father used to drink beer, but now he drinks sake.
Literal
Father [topic-は] previously beer [object-を] was-drinking [but-が], now [contrast-は] sake [object-を] drinks.
~ていた describes a past habitual state ('used to drink'). The contrast between 以前 ('previously') and 今は ('now' with contrastive は) creates a clean before/after comparison. 日本酒 is specifically Japanese rice wine.