While playing a game, press ~ to display the console. Then, enter "open [level name]" and press [Enter] to activate it.

Use one of the following level names for it:

AR-Asteroid
AR-Barracuda
AR-Luna
AR-Orbital
AR-Sequoia
AR-Sky
AR-Tut
Autoplay
Entry
Nerf
Nerf-Plaza
PM-Amateur
PM-Asteroid
PM-Barracuda
PM-Champion
PM-Luna
PM-Orbital
PM-Sequoia
PM-Sky
PM-Tut
PMX2-Luna
PMX-Asteroid
PMX-Luna
PMX-Orbital
RR-Amateur
RR-Asteroid
RR-Barracuda
RR-Champion
RR-Luna
RR-Orbital
RR-Sequoia
RR-Sky
SH-asteroid
SH-Barracuda
SH-Luna
SH-Orbital
SH-Sequoia
SH-Sky
SH-Tut