【Decrease】 , 【lessen】 , 【diminish】 , 【reduce】 , 【abate】 ,  【dwindle】 denote to make or grow less, but they are not freely interchangeable.

【Decrease】 and 【lessen】 are often employed in place of any of the others.

【Decrease】 normally retains, even in the transitive, an implication of the process of growing less, and suggests progressive decline.

【Lessen】 is a close synonym of 【decrease】 but the latter is to be preferred in contexts employing specific numbers; thus, it is idiomatic to say that a fever has lessened or that it has decreased from 101° to 99°.

【Diminish】 is a more precise word when the ideas of taking away or subtraction by an agent and of resultant perceptible loss are to be emphasized.

【Reduce】  adds to 【diminish】 the implication of bringing down, or lowering; it suggests more than any of the others the operation of a personal agent.

【Reduce】 also is applicable to lowering in rank, status, or condition.

【Abate】 differs from 【diminish】  and 【reduce】 in its presupposition of something excessive in force, intensity, or amount and in its strong implication of moderation or, especially when referred to taxes or imposts, of deduction.

【Dwindle】 , like  【decrease】 , implies progressive lessening, but is more often applied to things capable of growing visibly smaller. It specifically connotes approach to a vanishing point.