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
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!
Make your Designs in photoshop and enjoy your game with your way!
Always Remeber to put a transparent Background!