First of all you need an editor that can show/edit the data in hexadecimal format.
For hacking save files you need to know the exact number of gold/coins etc, save the file, copy it over to the PC and
look for the numbers in hexadcimal notation.
So if you for example have a 100 coins, you look for a value of '64' with the hex editor.
Numbers larger than 255 are split into multiple hexadecimal numebers like for example 345 becomes '0159'.
Sometimes the numbers are ordered in the opposite direction like '5901'.
If you are lucky, you can find the numbers pretty easy.
In some cases the data in the save file is encrypted so you won't find the numbers.
Also some save games contain a checksum to prevent easy editting.
But just give it a try.
Editting save games on the 3DS was pretty straight forward.
There are some homebrew tools like Edizon that can also do memory editting (not save file editting) directly on the Switch.
That is also a great way to start hacking games.