You use the Xacto as a very thin "wedge". Gently insert the tip of the Xacto into the space between the cab and walkway, and gently push inwards. (note - the mechanism under the "forward" end of the cab has space for the Xacto to go in much further).

This action will gently open a very small space in between the cab and walkway, into which you can then (as shown in following photo) insert small hobby screwdrivers, to gently pry the cab ends upward. See the other photos following to see where the "tabs" are at the four corners of the cab - understanding what these tabs are, and how and where they hold the cab onto the walkway, is key to understanding what you're doing here.