邊度有得換Casio DW5600錶帶？

This is easy. Just go to Javys to have the band replaced.
You can visit the company's Sheung Wan showroom or TST specialty store G-Factory:
http://www.javys.com/casio/contactus.htm
On your DW5600 the bands are held by internal double spring rods. Ask the staff if they can also give you that special tool that helps unlock the spring rods, so that next time, when your band breaks again, you can replace them yourself.
Some other stores (eg. those a Sino) may carry CAsio replacement bands, but the selections are limited. And of course they won't provide you with those special tools.