You use the owner's manual, specifically researching the five status lines on the fifth monitor.
The Manual Override is a deadman's switch lever, controlling the steam outflow vents, that automatically returns to the off position if no pressure is applied to the lever. The Manual Override cannot be controlled from the Core Management Room - you'd better be damn sure you want to manually override the steam outflow system. The Manual Override is safely out of the way of any steam outflow that would be released under normal circumstances. Do be careful when releasing the lever, as the speed at which it will travel in order to reset the Steam Outflow system is enough to cause fractures.
The Water Flow system has an automatic controller as well as a manual controller. The manual controller overrides the automatic controller if the water flow is increased for more than ten minutes, and the automatic controller overrides the manual controller if the water flow is decreased for more than ten minutes. Never reduce the water flow when the Steam Outflow system is being manually overridden, as this can lead to dangerously low levels of water in the system.
The Meltdown Sirens can be controlled from either the Core Management Room or from a remote location that is highly resistant to both heat and radiation. While the Sirens can be disabled, they will automatically re-enable if the Steam Outflow system is being manually overridden. The Meltdown Sirens are independently wired to the thermal sensors in the Core, and will sound a warning if the core temperature crosses the lower heat threshold of 1000*, and will sound a continuous warning in the unfortunate event of the core temperature crossing the meltdown threshold of 1300*.
The Core Controller Computer allows you to control the power output of the core, as well as the Automatic Water Flow System Target and the Meltdown Sirens. Caution is advised - operation at full capacity for extended periods of time runs the risk of damaging the power connections between your power core, the rest of the plant, and any location that obtains power from your core.
Your Gone Fishin' brand Nuclear Reactor Core is the finest in fission power generation. With a maximum capacity of 2 Jiggawatts (that is, 2 Jiggajoules per second), combined with our Anti-Chernobyl Meltdown Prevention System and our patented Downward Meltdown design, this is the safest Fission Reactor on the market! Even in the event of a catastrophic failure, the worst that will happen - short of explosives causing a core breach - is that you'll lose one reactor and that you'll be out of power for a short while. And really, is that so bad compared to Chernobyl, Three Mile Island, and Titan?
You use the terminal. At the prompt, you key in "Manual Override". The prompt returns a smart remark. "You're new at sabotage, aren't you?"