Board the dropship to escape the Sulaco | Mission 3: Sulaco Falls A: CM Guide

Notice that Xenomorphs have taken the place of the mercenaries. Despite what it looks like, you don't have to hurry to help them; the marines will have no trouble getting rid of the aliens. If you want to reach the dropship as quickly as possible, find the spot from which you can jump down below. It's quite risky, though, because on your way to the evac zone you may be attacked by Xenos.

If you don't want to take unnecessary risks, use the upper gallery to go around the whole hangar bay and reach the spot from which you've released the first arm. You can use the stairs again. Regardless of the choice, approach the dropship to prompt a cutscene that ends the mission.