You may not be able to change the pipe (or duct) height if you haven't imported a DXF file
The following section is from the Netsys manual:
If you do not have a DXF file, you can specify "Blank sheet," which means that you will have a completely empty drawing (ie a blank sheet) when you start using Netsys. For blank sheet projects, all drawing is strictly 2D, all components are placed at a height of 0m, so vertical pipes and ducts cannot be drawn.
If you wish to trace a true 3D pipe model then you would need to start a new project, and import a DXF background so that scaling of your pipework layout is accurate.
If you wish to use a 2D model, then you can trace your project isometrically to emulate height changes. Different Isometric grid angles are available in the defaults menu.