How can I change the shape of my jawline?

I suggest posting pictures , but most often I'm using fillers or fat to reshape the jawline without the need of implants.  Best, Dr. Emer.

Depending on what exactly you want to enhance or soften about your
jaw-line there are a variety of options. One strategy to narrow the width
(roundness) of the jaw is to use Botox in the masseter muscles to decrease
their contribution to your jaw width. The masseter muscles are chewing muscles,
but Botox treatments have little to no effect on your ability to chew or eat.
It is a popular strategy in woman of Asian descent and can be done very safely.
Other options include fillers, fat, or implants to enhance areas of desired volume
in order to soften the roundness you describe. Your best bet is to arrange a
consult with a Board Certified Facial Plastic Surgeon who can discuss your
specific goals.

Only implants, particularly chin and jaw angle implants, can change the shape of a jawline from round into a more defined one. The jawline can not be successfully changed on a permanent long-term basis without surgery.