Hex editing is like typing
';djhgkjhsdkh
That was me successfully striking keys on a keyboard but it was meaningless in the long run.
A hex editor is then a tool which just shows you the hex that every program and file we see boils down to. It will usually come with three sections, possibly plus a few more to expand upon what you have selected/highlighted.
Usually on the left will be the location in the file/disc you are viewing. In the middle will be the data and on the right will be the data decoded as text of some form (usually ASCII but you can change this). You edit in the middle section or the ASCII section if you think it helps. The middle section will be comprised typically of hexadecimal (hence hex editor) and thus means you have values 0 through to F (the hexadecimal/base 16 number range) to enter as each character. Other than insert or overwrite mode (much like a word processor there in that you either insert data and shuffle everything after it forward or you overwrite it and everything after stays where it is) there is not a lot else to the basic usage.
Doing any kind of useful edit to a ROM/program/file requires you to know the context in which you are editing and that can be anything in computing and beyond. Also as we measure things in megabytes and more these days (a byte is two hex digits, a kilobyte is 1000 or 1024 of them, a megabyte is 1000x1000 or 1024x1024 aka a millions) then it gets very tedious if you are going to edit lots in a hex editor so nobody does it if they can help it, or they will do it all externally and then put it back in.
Short version is it sounds like you do not want to learn hex editing but ROM hacking instead. Fortunately we can do that
http://gbatemp.net/threads/gbatemp-rom-hacking-documentation-project-new-2016-edition-out.73394/ . I am not sure I would suggest learning on the 3ds but if it is what motivates you then there are worse options.
