Press ~ and type "sv_cheats 1", then "restart" to enable cheat mode. While the mission restarts, press ~ again, then enter one of the following codes:
Invincibility: god
Get some weapons: impulse 101
Flight mode: fly
Free movement: noclip
Enemies cannot see you: notarget
Spawn indicated item: give [name]
Kill all bots: bot_kill
Restart map with goals intact: restart
Change map: changelevel [map name]
Restart round: sv_restart 1