Jar file is a zip archive that can be opened with any file archiver tool. MANIFEST.MF is plain text file in MANIFEST folder, contains info about name, author, version of application. It could be simply edited with standard text editor program. Logo files logo.png
could be created or edited with image editor.
Using online translation tool you can read these articles about modifying Gravity Defied: http://www.dinedi.ru/publ/.
Actually, it's very simple to edit game without knowledge of any programming language.