The Forerunner 645 takes any generic watch strap that has a band width (also known as Lug Width) of 20mm.

The easiest way to identify this is by either looking on the underside of your watch which should show the model name or model number (010-01863-10 for the Forerunner 645). Alternatively, the app on your phone that the watch connects to will usually tell you what model you have.

These can be easily changed by removing the pins on the watch strap and attaching your new strap. The video guide below has a great guide on how to change these wristwatch bands.