You cannot directly change the default units of measure.The only way to change to metric is to have the portal administrator use the Portal Services REST API to alter the portal's region to be something other than US or a null, as all other regions use metric measurements. Be aware, though, that altering the region for the portal also affects the default extent of new maps (if you're using the default basemaps) and will result in some website headings and details appearing in the language of the region you have chosen.So, if you are in the US and want to use metric, the portal admin could set the region to WO (for World) and your headings and such will stay in English, but the default basemap extent will be the world (which it is anyway). For other countries, the admin can set the region to whatever region applies. For example, if you're organization is in Iceland, your admin can set the portal's region to IS.
