Oops, I should have mentioned I grabbed the screenshot after I edited the file. You original file has 04 / box since your hero had 4 skill points; I added 100 points (decimal value 104 is hex 68, which has the ASCII value of 'h').
Once you have the value selected, you just need to click where you want to edit on the hex side of the display if it does not have focus (in XVI32 the side with focus has a white background), and then type in the new value (68 in this case) to overwrite the original. Like text editors, hex editors can be set to insert new characters or overwrite existing character, the latter of which is usually the default (in XVI32 the edit mode is shown on the status bar at the bottom of the screen).
You pretty much have it; in XVI32 at the sage you made the screenshot, all you have to do is hit '6', '8' and save (or for a different number of skill points, use Windows Calculator in advanced mode, select 'decimal' if it is not already, type in the desired amount, then switch to 'hex' to convert it).
The values shown on the ASCII side of the display may or may not mean anything. If there are text headings, etc. in the file there will be ASCII characters, otherwise it may just be coincidence that some hex value just corresponds to the ASCII value of a letter.
The boxes are there for any hex value that does not have a corresponding ASCII value (in the options appearance tab you can switch XVI32 to use a blank, which I find looks better / clearer).
