To use on your smartphone, you must first have a suitable navigation app installed that can store offline topo mapping - such as OSMaps, Outdoor Active, Anquet OMN, Memory Map, or Topo GPS. Walkhighlands files work with all outdoors GPS apps.
The link above will download the file to your phone's downloads folder. You may be able to then open it in your installed navigation app by simply clicking the file. If you get a 'Cannot Open File' message, then after downloading you need to open your phone's File Manager app (Files on iphones), go to the Downloads folder and open it from there.
If you are still having difficulties, contact the support for the navigation app you are using. We haven't used all these apps ourselves and cannot offer support on them.
To use the GPX file on a dedicated GPS device, you will need to download to a computer first, and then copy it to your GPS device. See your GPS manufacturers instructions for how to do this on their device.