You want to expand the metal of the headband outward, and yes a bit upward, like you’re bending it to be less tight. There’s lots of other guides out there about how to loosen headband clamping pressure, and of course, if you do it too hard, you might break something. So do it at your own risk!
A shoebox, basketball, soccer ball, or other head stand-in works well if you don’t want to bend it with your hands. You just extend the headphones and place them over the box/ball for 24 hours as if you were putting them on your own head. The box/ball then accomplishes the task of gently relaxing the clamp force the way it would if you wore them for hours yourself. Most headbands lose a little pressure over time, you’re just accelerating the process and not using your own head.
ZombieX’s famous guide does a great job of showing the minute differences between the DT770 models.