(1):

(v. t.) To exchange; to put or substitute something else in place of, as a smaller penalty, obligation, or payment, for a greater, or a single thing for an aggregate; hence, to lessen; to diminish; as, to commute a sentence of death to one of imprisonment for life; to commute tithes; to commute charges for fares.

(2):

(v. i.) To pay, or arrange to pay, in gross instead of part by part; as, to commute for a year's travel over a route.

(3):

(v. i.) To obtain or bargain for exemption or substitution; to effect a commutation.