Instructions
Download iFile from the Cydia store. This program allows you to browse and edit files on the iPhone. Note that the iFile app from the iTunes store differs from the Cydia app.
Open the iFile program. Tap "Home," then "Applications" and scroll down until you find "Doodle Jump." Tap the Doodle Jump folder.
Tap the "Library" folder, then "Preferences." Tap the file titled "com.yourcompany.DoodleJump.plist". This file should be at the bottom of the list.
Tap "Edit" in the top left corner of the screen. Scroll down to the "DoodleJumpHScore" section. The current Doodle Jump high score should be printed directly below this section.
Delete the current high score and replace it with any number of your choice.
Scroll down to the "DoodleJumpLocalHScores" section. Delete the number from the top of the list and replace it with the same number from the previous step. The "DoodleJumpHScore" and "DoodleJumpLocalHScores" numbers must match in order for the modification to work.
Tap "Save" and close iFile.