EDIT: Just a heads up, reading variable name can often spoil the story so be careful before you do this.
Open the game and load up your save file. Press Shift + D to bring up the dev console and select Variable Viewer from the list. You'll have to scroll through the list and find the variable that stores your player name. Once you've found it, press Shift + O to bring up the console and type in
Code:
variableName = "Whatever name you want in between these quotes"
For example in Sensei Overnight the player's name is stored in a variable called 'player_name' so to change it I would type in: