Apple

Which Apple Watch do you have?

This states, on the back engraved.
(top right)

Here you can see which cabinet size you have.

This is indicated in millimeter,
for example: 38, 40 or 42mm etc.

How to change Apple Watch strap?

How to change an Apple Watch strap is very simple. You click the button, slide the old strap out, and click the new strap in.