。
She sang better than usual.
Literal
She [topic-は] usual [than-より] skillfully sang.
より marks the standard of comparison ('than X'), and the comparison is implicit — Japanese doesn't need an explicit 'better' word: 上手に歌った compared to ふだん simply means 'sang in a more skillful way than usual.' ふだん ('ordinarily, normally') is a noun-like adverb, in contrast to ふつう, which leans more toward 'as a general rule.' 上手 is a na-adjective; its adverbial form takes に — 上手に — to modify the verb 歌う.