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if you're talking about warmups, just perform a trap exercise like overhead press at a lower weight
if youre talking about stretching the muscle out, the trapezius is made of 3 different sections, but only the top portion of it (neck region) is capable of any real stretching. try flexing your neck forward and down and around
as for the other portions of the traps, its pretty difficult to stretch those as they are resting on top of back/spine muscles that are much tenser and therefore prevent any real stretching to occur
hope that helps