The question I want an answer to is: How do I upregulate my RMR as a person who's been borderline underweight on much less than my RMR should be for my age/sex/

The question I want an answer to is: How do I upregulate my RMR as a person who's been borderline underweight on much less than my RMR should be for my age/sex/weight? I don't believe this adaptation to saving energy comes without costs, especially cognitive, and I don't want to pay them.