One is to change the length of the tube, and thus the length of the vibrating air column.

NPR: Young Tuba Player Gets Nod from Phila. Orchestra

It's possible to change the length of the entire tube on a natural horn, but only by adding lengths of tubing called "crooks, " or by switching one crook for another.

NPR: Young Tuba Player Gets Nod from Phila. Orchestra