John Gill Commentary Numbers 20:25

"Take Aaron and Eleazar his son, and bring them up unto mount Hor;" — Numbers 20:25 (ASV)

Take Aaron and Eleazar his son
His eldest son, who was to succeed him in the priesthood, and did:

and bring them up unto Mount Hor ;
to the top of it, they being now at the foot of it, where the people of Israel lay encamped.