How do you open a staple gun?

No they don't. What actually happens is a hammer hits the staple from behind, the hammer is the same size as the staple to stop any distortion. The method by which the hammer is moved depends on the...

First open the top of the stapler then put the staples in the top. Finally shut the stapler.

Normal store opening hours. For local information try their website. They may have a local store finder which you can consult

I usually put the staples in the stapler point down.

Yes. Staples is open on Labor Day for its normal weekday hours. For the Staples at Penn Station in NYC, that means it's open until 8pm tonight - late enough to get home from Labor Day festivities and...