How would I access my 2012 MacBook Air's Hard drive?
This site claims that the drive is not officially user replaceable, but it’s simply a case of the connector being different.

But you say it’s the Mid 2012 model 4,2? That is the model number for the Mid 2011 model as per the specifications listed here. iFixit has a good rundown on how to get to that one. And Other World Computing sells and external enclosure called the Envoy that can take that internal SSD and make it an external USB 3.0 drive. Instructions on how to install a drive in that Envoy enclosure are here as well.