Most 13 year olds won’t follow “traditional” exercise workouts.
You say “make a significant changes for her”? Does she want to change?
I am assuming her parents hired you, I would speak with them and find out if they are pressuring her and then speak with her alone and narrow down her goals and motives.
I would get the entire family involved if possible.
Find a sport or activity she likes such as swimming, tennis, bike riding,dancing, martial arts, jump roping etc and work off of that. If she already detests exercise at the age of 13 this will be a challenge for you in many ways.
Think in terms of short attention span, keep things simple and fun.