How to hex-edit save files in JA2 1.13 (since there's hardly any information to be found on it online), with focus on fixing bugged attributes in a save file if you don't want to go back to an earlier save, accept the new values or ditch the affected merc, and with an assumption that you don't have much experience in hex-editing:
1. Google for a free hex editor if you don't have one (I used HxD). You should also grab some convenient hexadecimal-decimal converter, or use an online one, and convert to hex all the in-game attribute values of the merc you're focusing on.
2. The save folder should be in \Profiles\UserProfile_MODVERSION\SavedGames (SaveGameXX are the nameable saves).
3. Open with the hex editor the save you want to modify; you can find sections on each merc by searching for a "text-string" that is their name or nickname.
4. Somewhere between their name and the next merc's name, there are 1-byte (two digits, e.g. 00 or FF) hex values of both the purely-cosmetic +X increase that you see in Personnel Manager laptop tab, and the actual attribute/stat values (at least in my version the stats appear to be roughly spread in the proximity of a chain of ten FF values, but supposedly the save structure keeps changing with different mod versions, at least according to the numerous answers as to why there are no updated save editors around); there's also stuff like xp values for attributes, but good luck knowing the exact numbers (for a rough estimate, they won't be higher than the levelup thresholds set in mod's config .ini file).
5. You can copy-paste the whole segments of either two different mercs from the same save file, or the same merc from two different save files, into a text compare program to highlight changing values to make finding stuff easier (in my very brief search for hex editors, I haven't found any with a satisfying comparison feature).
6. Updating the +X increase is merely changing that particular hex value, but modifying the stats results in a save that won't load, which reportedly is because of the game using an anti-cheat checksum somewhere in there. The checksum is 4-byte long and can also be found in merc's segment (I found mine roughly in the middle of it).
7-a. If you can figure out how to update the checksum, likely basing it on the code of
this save editor for the vanilla game, then you should be able to do whatever you want with the stats.
7-b. Otherwise, you want a different save where the merc in question has the stat values you want (preferably you already used it for the difference comparison above). Find all the differences in stat values in the second save, fish out the checksum, then copy-paste them onto the stat and checksum values in the bugged save file. If you don't have such a save, I suppose you might try starting a new game and recreating merc as an IMP, or temporarily modifying non-IMP merc's starting stats in relevant mod files, saving, and extracting the hex values from that save.
8. Save the changes to the save, launch JA2, attempt to load that save.
9. If you did everything correctly, the game will let you load the modified save; you won't
see any changes to stats (except for the increase modifier thing in the laptop), but that's just the stat display not getting updated and it'll fix itself on the next levelup (separately for each of the changed stats); the values for the display don't appear to be in the same area as the merc segments (the issue also occured when I copy-pasted the whole thing).
10. If you want to make sure it worked, you can lower the xp thresholds for the changed skills in mod's settings (e.g. if one-third of the progress bar for a skill is filled, lower the subpoints_to_improve for it to below one-third of the value that's in the settings), load the save, go to tactical view, and after a short while you will get stat increases that will update the display values. Restore the threshold values, reload the save, the end.
An example output of text comparison (using diffchecker.com):
Red is from a bugged save, green is from an older save with correct attribute values. Additional yellow CTRL+F search for one of the bugged attributes (63, which we want to change back to 2E). 61 vs 5C a bit above it and to the right is another bugged stat. Further to the right, 3D vs 08 is the +X increase modifier for the first bugged attribute. The checksum is at the bottom.
I haven't checked if the inventory is stored there too (the merc happened to have the same loadout in both saves), but in case it is and makes finding stat changes harder, you could temporarily drop everything the merc is carrying in both saves and re-save them. I also am not up to date with the recent 1.13 builds (or whether someone finally smuggled in an updated save editor in there or removed the checksum from the save structure) but it should nonetheless be applicable to any of the older and/or "stable" builds that are not compatible with the old save editors.
Feel free to take it further.