How can one instruct a person to die?
Seforno: Hashem ordered Moshe (not to die, but) to accept death as an atonement for his sin. 1
Oznayim la'Torah: Hashem meant that he should prepare himself to die in the same way as Aharon died 2 - by climbing on to the bed, lying down on it and stretching out his hands and feet.
Presumably, that is why Targum Yonasan translates the word as 'Sh'chov' - lie down, as that is what Moshe had to do to prepare to die. Hashem did the rest.
Why did Hashem mention Aharon's death?
Perhaps it is in response to Moshe's fervent Tefilah 3 to postpone his death until after he had entered and seen Eretz Yisrael. Hashem therefore answered that, since he and Aharon were both involved in the same sin, 4 it would not be correct to grant him a temporary reprieve now that Aharon had died.
Rashi (in Chukas Bamidbar, 20:26 & 27): After taking off Aaron's clothes one by one and placing them on Elazar his son, Moshe told Aharon to lie on the bed (in the cave on Hor ha'Har), to stretch out his legs, shut his eyes and close his mouth. He then left the cave and declared 'How praiseworthy is he who dies in this manner!' Refer also to Bamidbar, 20:26:1:1.
See Ba'al ha'Turim.
See Targum Yonasan.
As the Pasuk specifically states.
