“You must take the stairs.”

-or-

“You must use the stairs.”

The word “take” is used in the same sense that one would “take” a train, a plane, an automobile to travel. You “take” the stairs. Honestly, I rarely hear someone use the phrase “climb the stairs” in a directive manner.

Edit: harleycharley proposes that “take” cannot be used properly in that sentence. However, the English word “take” has a very broad semantic range.

The Dictionary.com Unabridged (which is based on the Random House Unabridged Dictionary) lists 106 definitions, and that does not include verb phrases or idioms. Number 53 reads as follows:

to adopt and enter upon (a way, course, etc.): “to take the path of least resistance.”

If you can “take” the path of least resistance without removing it or changing its location, then you can “take” the stairs as well.