How can I start over?

If you completely close out the game and then open it again, you get the "clean start" that you are looking for, with the broom closet unboarded.

Things This Will Reset

• The Broom Closet
• The "Break Room" commmentary
• The Boss's Office, including accessibility to the washroom and elevator, as well as the door-opening setting.
• The initial rooms (though any random setup can occur after the first "standard" start)
• Your visual orientation at the start (if you get an ending or restart, you will always be facing out the door, but at the very start, you are facing your monitor)
• The "Serious Room" dialogue.
• The "Employee Of the Month" picture.

Two things of interest to get a cleaner start.

First, the save files are, according to this gamefaq thread in

...saves are located in your steamlibrary \SteamLibrary\SteamApps\common\The Stanley Parable\thestanleyparable\save then the subdirectory you ll find there.

Deleting them will make sure nothing from previous saves might be used.

Second, this youtube video shows that you can reset your achievement progress with

Use the console command "tsp_resetzaxis" to reset your achievement progress

so that you can do them again.