There are several different ways you can make adjustments to a neck pillow to suit your preferred level of firmness and comfort. You can add or remove some fillings to the pillow like in the Sweetnight Shredded Memory Foam pillow.
Another easier way is choosing the right contour or side. There are pillows such as the Perform Memory Foam pillow which can allow you do this. They are double contoured and allow you to pick the side that’s more comfortable to your sleeping style.
Removing a layer of the foam is also an easier way of adjusting a pillow. Some pillows come with three different foam layers where you can remove the middle layer to reduce the height. A good example is the UTTU Sandwich pillow.