Access Editor

From PreyWiki

Unlike previous 3D Realms titles, the editor for Prey (known as Preditor) is inside the game itself, it is not a seperate program. To access the editor, you will need to manipulate your Windows shortcut.

You will need to add this text to the shortcut target in addition to the text that is already there.

+r_fullscreen 0 +vid_restart +r_multisamples 0 +com_allowConsole 1 +editor'

When you make these changes, your shortcut should look something like this, and will launch the editor and not the game.