If its a one time job, trial and error is the fastest way to get results using HxD.
First you need to be clear what to search for. Money / EXP / Items and so on...
Increasing/Decreasing values are easy to spot, but the game can use
protection like crc to make sure the file is unchanged.
Make 2 saves with different values,
decrypt both saves and compare them.
(A few games change offsets for each save slot > use the same slot, export step by step)
If you are lucky, the value is simple DEC > HEX and a fast hit.
Sometimes they count values different like 1 = 4 , 2 = 8,
a full compare might show more than one possible
results and testing is necessary.
Change value and import the save back to 3ds.
If the savegame is now corrupt, it might be protected > if not, gooood.
For Items, it can be tricky! A few games have more than one Byte
just counting it, there might be another Byte for picking it up yes/no.
At least this should be faster than to analyse the games code
