You're going to have more than one so simply follow the wires from one that you find to the next. Some are push button re-settable while most reset on their own. Many times they go bad and need replacing.
and btw, the high temp tripping is not your problem, it's doing it's job of protecting you. The furnace overheating is the issue, lack of air flow (dirty air filter, perhaps) or poor air circulation of some sort is the reason the limit is tripping.
You don't close it. You find and correct the cause of it opening. And then you replace it. When an auto reset high limit doesn't auto reset anymore. It has opened too many times and is no longer reliable.
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