Rashi on Numbers
ודשנו את המזבח AND THEY SHALL TAKE AWAY THE ASHES FROM THE ALTAR — from the brazen altar.
Siftei Chakhamim
They shall take the ashes. Not that they should place the ashes there (as the word ודשנו might imply).
Rashi on Numbers
ודשנו means, they shall remove the ashes from off it (cf. Rashi on Exodus 27:3).