Rashi on Leviticus
והזה עליו מן הדם AND HE SHALL SPRINKLE UPON IT WITH THE BLOOD — After he has made with his finger the application of the blood upon the horns of the altar he shall make seven sprinklings on its top (עליו signifies "upon it") (cf. Sifra, Acharei Mot, Chapter 4 12).
Siftei Chakhamim
He sprinkles seven sprinklings on its top. Because it is written “on it,” implying that he placed [the blood] on its roof.
Rashi on Leviticus
וטהרו AND THUS HE CLEANSES IT from the defilement that happened to it in the past,