How Can I change speaker?

Ifixit has a service manual that gives instructions on how to remove speakers from your XPS 15z at the link below. The speaker removal starts at page 44.

xps-15z_service-manual_en-us.pdf

As for where to find parts, I would suggest you remove your speakers to verify the part number, and then just do a search in your browser for your part number. That will bring up a lot of options. You can get a general sense of price of the speakers if you do a search for "XPS 15z speakers"; but, again, I recommend verifying your part number before ordering anything.