Save files can be altered via text editor, although some of the advanced editing requires some bit of experimentation and, unfortunately, playing the game some first. What I mean by that last bit is that some of the data written to the saves are not in fact written in until they are relevant to that save's progress, such as perks. Surveyor, for instance, is written nowhere in a save that does not have it unlocked, but unlocked it will add the line ""SURVEYOR": 1" within the Perks group. Health, Stamina, and Mana uniquely requires you to alter the tiers the same way you alter their relevant stat. So if you wanted to change the health to 600 from the base of 60, for example, you need to find "healthTiers" found above health and alter the tiers in the same way, so you'd change all four of the "15" values to "150" so they add up to the health total.
I'd recommend backing up a copy before going nuts and diving into save editing until you've gotten the hang of it and also I'd recommend using Notepad++ as it caches your file edits so you can use "undo" a lot more flexibly than regular notepad, even after saving changes to the document (closing N++ clears the cache, though, so keep it open until you're satisfied with the edits). The save files are data sensitive to the point that anything out of place, a single missing space or misspelled word or accidentally removed bracket will make the file unreadable until the offending errors are corrected or removed. N++'s undo permits a greater ability to deal with that fact since you can undo the last set of changes if they cause the file to fail so you don't have to start all over with a new save entirely. Something else to remember is that save slot 1 is named save0.json and save slot 2 is save1.json and so on.
