AFAIK, the only way to do this is through a plugin.
You have to create a file with the name {RemapKeys.js} and put it into the folder {FemuZero/js/plugins} with the following content:
JavaScript:
Input.keyMapper[87] = "up";
Input.keyMapper[65] = "left";
Input.keyMapper[83] = "down";
Input.keyMapper[68] = "right";
And then you have to edit the file {FemUZero/js/plugins.js}, adding the following line:
JavaScript:
{"name": "RemapKeys", "status": true, "description": "", "parameters": {} },
It has to be somewhere between the "[" and "]". If you add to the last line, you have to ensure that the line before ends with a comma, if not, you have to add it. Here's an example:
View attachment 4457556
Making an error while editing this file can cause the game to crash on startup, so I recommend making a backup before editing it if you're not sure exactly what you're doing.