To change the value of a decimal variable, use the EVAL debug command.
CL declarations: DCL VAR(&DEC1) TYPE(*DEC) LEN(3 1) VALUE(73.1)
DCL VAR(&DEC2) TYPE(*DEC) LEN(2 1) VALUE(3.1)
Debug command: EVAL &DEC1 = 12.3
Result: &DEC1 = 12.3 = 12.3
Debug command: EVAL &DEC1 = &DEC2
Result: &DEC1 = &DEC2 = 03.1