Open Undertale
Go to the game window
Press F4
If F4 does not work, press Alt + Enter
If needed, open the in-game settings and enable fullscreen
If the game still opens in windowed mode, check your display settings or launch options
Open Undertale
Go to the game window
Press F4
If F4 does not work, press Alt + Enter
If needed, open the in-game settings and enable fullscreen
If the game still opens in windowed mode, check your display settings or launch options