Press F4
If F4 does not work, press Alt + Enter
If you are on Steam, right-click Deltarune in your Library, select Properties, and check the game’s launch or display settings
If the game still will not fullscreen, change your Windows display resolution to match the game’s resolution
On Mac, use the green window button or the game’s fullscreen shortcut if available
On Linux, try F4 or Alt + Enter, and check your window manager’s fullscreen shortcut
If you are using a controller, open the game’s settings and look for a fullscreen option if available