Step 1: Locating the App Folder
1) Go to /var
2) Then Go to /var/mobile/Applications
3) Search for "Hill Climb" and open it.
4) Now Open "Hill Climb Racing.app"
Step 2: Searching for the Required File
5) Now Search for "Car.png" File
6) Design your own car in Photoshop and Name it "Car.png"
7) Replace it with the orignal file in that folder and similarly for other files in that folder can make a all new game.
Step 3: Design Other Things Too!
You can see that Similarly i also made many designs and changed with stock which have made it cooler!.
Make your Designs in photoshop and enjoy your game with your way!
Always Remeber to put a transparent Background!