OK, so this won't be a complete how-to, but it'll help you somewhat.
Note that the data you seek will be available as a property of a CMAltitudeData object. So, first we need to get that data from a CMAltimeter object. Here's a link to the CMAltimeter class reference: https://developer.apple.com/library/ios/documentation/CoreMotion/Reference/CMAltimeter_class/index.html#//apple_ref/occ/cl/CMAltimeter
This is where I stop - my intent here is not to guide the reader through the process, but to give the reader a place to start.
Note that the data you seek will be available as a property of a CMAltitudeData object. So, first we need to get that data from a CMAltimeter object. Here's a link to the CMAltimeter class reference: https://developer.apple.com/library/ios/documentation/CoreMotion/Reference/CMAltimeter_class/index.html#//apple_ref/occ/cl/CMAltimeter
This is where I stop - my intent here is not to guide the reader through the process, but to give the reader a place to start.