expression, godan verb (-ru)
go up the stairs; climb the stairs
Common phrase for ascending stairs. 上がる is used here as a transitive verb with を, meaning to move upward along the stairs.
エレベーターが混んでいたので、階段を上がった。
The elevator was crowded, so I went up the stairs.
彼は一気に階段を上がってきた。
He came up the stairs in one go.