The os is free to upgrade for one year, after that a subscription fee will follow. But Windows 11 will be out then! apps market will likely to grow, and with apps the hardwares, licenses, advertizing, corporate marketing.... it's not the 90's any more.
with arcgis, you can use one of toolbox's tool, called surface volume, but first you need to convert contour line into surface data, it is basically raster data
you can convert it with conversion tool from arcgis.
hope it helps