I have a Java executable jar file which is currently desktop app. My old computer got corrupted and so did the source files. How do I edit the jar?

Asking this questions on behalf of my friend :

Qustion 1 ) I have a Java executable jar file which is currently a desktop app. My old computer got corrupted and so did the source files. How do I edit the excutable jar file so I can include the latest GUI?

Question 2) Thereafter, how do I make it a SaaS as a B2B solution in AppSource ?

Haven't tried anything yet.