For manual editing, what you'll need to do is configure and implement a test system on the version you are aiming for, populate it with data in such a way that you've got an example for each type of data you've got in the source system, export the xml, and then work out what storages have changed and how, then exit the old xml to match the new one.
What I'm getting at here is that yes, it's technically possible to edit the xml. It's effort intensive, hard work, slow and prone to mistakes and problems. Compared with doing the upgrade properly, as Onkar pointed you to, it's a nightmare. I certainly wouldn't even bother - if the backup data is small and simple enough that I'd think about doing manual editing, it's also so small and simple, it would be far quicker to type the data into the new system
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