I used Inkscape to edit the svgs, it can’t be done from within Fritzing AFAIK. It took me around a year to go from never heard of and svg or Inkscape to being able to make parts as the learning curve is quite steep (inkscape alone is quite complex). Here are pointers to a couple of tutorials which apply to the current version of Fritzing (most others I’ve seen are for earlier versions):
Other than that, your best bet is to try it and ask (preferably posting the .fzz or .fzpz file that doesn’t work) here for assistance (thats how I learned).