Like others have said, using boiling water is the best way to do this.
You can either remove the straps and immerse the whole strap in boiling water, or carefully hold the end of the strap in the water while also holding the rest of the chest piece. Make sure to wear gloves to prevent steam from scalding your hands. Try shorter amounts of time at first (20 to 30 seconds) and shape accordingly, gradually increase the amount of time for the immersion if you need the should strap to be more malleable. Just recognize that each additional immersion counteracts the shaping work you previously performed.