Things You'll Need
Instructions
Install and open Hex Workshop. Go to "File," then "Open," then navigate to the "Diablo 2" saves folder. This is typically under "C:\Program Files\Blizzard\Diablo 2\save." Open a savegame.
Click the "Edit" menu, then select "Goto." Make sure "Hex" is selected, as well as "Beginning of File." In the "Offset" form, type or copy the desired hex location. Ignore the "0x" portion of the hex address. As an example, the "Magic Arrow" skill for Amazons is at location "0x26F."
Type or copy "26F" into the Offset form. In the right panel, a value should be highlighted, corresponding to the hex address highlighted in the left panel. You can edit skill values from 01 to 14. Anything outside that range may corrupt the savegame.
Go to "File," then "Save". This will automatically overwrite the savegame, but will also ask if you want to back up the old savegame to a .bak file. If your edited savegame becomes corrupt, it can be deleted and the backup can be restored by changing the extension from .bak to .sav.