Entropy can only be decreased in a system if

heat is removed, because entropy is the thermodynamic quantity representing the unavailability of a system's thermal energy for conversion into mechanical work, often interpreted as the degree of disorder or randomness in the system

Heat is removed

Explanation:

Entropy is measure of chaos or disorder. Entropy of the universe is increasing that means it is positive. Entropy is positive for irreversible process. For entropy to be negative which means for it to decrease, heat must be removed  from the system. It will reduce the disorder of the system that is this process will bring order in the system and hence, the entropy would be negative.

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