Basically by pushing in the circuit brakers:
cb G/N COMPUTER MNA - close
cb G/N COMPUTER MNB - close
...and just in case you don't know what I'm talking about, you can use Project Apollo Checklist MFD to show it to you:
http://www.ibiblio.org/mscorbit/mscforum/index.php?topic=2252.0, the part below "to watch the show in Virtual AGC mode do the following:"
Cheers
Tschachim