It’s actually pretty easy. Tape your pattern to a large paper and start drawing an outline around it. From the outline, slide your pattern up 3″ or whatever you want your measurement to be and redraw the hemline. Replace your pattern to the original outline and retape. Fold up the bottom of the pattern though so you can see where your new hemline should be and cut your paper on this line. The only change that’s been made is that the hemline has been brought up. Definitely make a muslin though so you can test the length and make sure you’ll be happy with it. Good luck!