Instructions
Leaving the building will only be available after you complete the eighth wave of zombie attacks. To complete waves one through eight, shoot any zombies attacking, build windows and purchase higher calibre weapons with the money you earn.
In the beginning of wave nine, run to the nearest window and destroy the fortifications on it. You can do this by shooting at the wood with any gun, or by smashing the fortifications with your knife.
Stand against the open window and look down at the ground.
Jump up and down while continuing to look at the ground. You must do this repeatedly 11 to 15 times without interruption from a zombie attack or looking up. If you are interrupted start again in the beginning of the next wave.
Once you are successfully on the window ledge, go through the window. You are now able to run outside and climb to nearby roofs where the zombies cannot reach you.
When you run out of ammunition, you have to re-enter the building to purchase more. Once inside you must complete the same series of tasks to exit the building again. The best time to do this is at the beginning of a new wave of attack as there are a few moments before the zombies arrive.