I'm not sure of the context of your question, so my comments will reflect that.
Retro fitting simply means re-applying changes to an updated version. If you make changes in the existing JDE version, and then upgrade to a new version, some of those changes can be lost. As such there is no "step by step..any document with screenshots" to show how it is done. Retrofit is done the same way the original change was done, whatever that change was. The most common retrofit is changes to standard objects. For example changes to event rules for a standard application or UBE.
If you are tasked with retrofitting changes made by others, you will need to know what those changes were and how they were made in the first place. This may make it necessary for you to have training, or at the very least read the appropriate manuals - that is where you will get any "step by step..any document with screenshots".
I've done a retrofit in JD Edwards, its not as bad as you think. For most Applications and Reports you can do a ER Compare if the system is setup that way. If not you can use Object Browser to export the code from the 2 environments and use a tool called winmerge to highlight the differences and then you can apply the changes that way. Tables will need to be manually check. Also you will need to build your business functions.
ER Compare is nice because you can merge to the new object with a click of a button.
I might have missed some but i'm sure someone can chime in.
Someone setup our environment where our PD 9.1 was accessible in 9.2, the setup that was used I am unsure of but essentially I would checkout the object do a compare and if there are customizations I would evaluate if they are still needed, if they are still needed then I'd bring it over.