Depends upon the monitor that is in your Astro. If you keep the machine off and unplugged for the night, it shouldn't be to dangerous to perform the task the next day. Take the back plate off for easy access to the PCB. Look for the yoke lines (4 wire plug from tube to PCB) and should see a similar looking plug on the board next to it. Use a flashlight to ensure its the 15khz plug as I believe there is also a reverse (mirror or upside down) plug.
Saftey notes: don't touch the tube yoke (where those 4 wires are soldered to the tube neck) and unplug/replug on the pcb with just one hand (keep other hand behind your back) you are pretty much safe.