How did the geek squad agents changed my laptop hard drive without me losing my data?

So my 4 month old laptop speakers got busted but i didn't do anything to break the speakers though and the laptop was in good condition. I took it back to best buy geek squad a week ago and they replaced my speakers and they also said they changed my hard drive and it got back to me today. They mention that they changed the hard drive verbally and on paper and I'm amazed at how they changed it without me losing my data. How were they able to do that ?? It's amazing however they did it. It looks just like when i left it.