Most of the time if you click on the units it will toggle between mm and in. Parts editor was new and not finished when development stopped in 2015/2016, now development has started again there is work being done on parts editor but it isn’t released yet. You may find this set of tutorials (one section of which explains why I don’t use parts editor much) of use:
unlike most of the others these two apply to the latest versions of Fritzing.