You need RPG Maker VX Ace and RPG Maker Decrypter. (Google is your friend)
Move the Game.rgss3a from this game into RPG Maker Decrypter, open it up and load it.
Extract all files and copy and paste those files into this game folder.
Click on Generate.rvproj2 button at RPG Maker Decrypter, copy that new file into the game folder as well.
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Open the Game.rvproj2, it will open RPG Maker VX Ace with the game files loaded.
Click Tools -> Database -> Actor
Change the main character name to whatever you like. I changed to Maou in this case.
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Note that this will change the name of the character as he speaks / on the menu, etc. But if anyone mention his true name Voice in any text / conversation it should still be Voice, since that string is in the event itself. However they often refers to him as only Maou / Maou-sama, etc
PS: The actor name is saved into your game, so you would have to start a new game for this to take effect.