Before we begin, identify the size of your Apple Watch and, by extension, the size of the band you will require. The Series 4 and Series 5 versions come in 40mm and 44mm sizes, whereas the older variants (Series 1, 2, and 3) come in 38mm and 42mm sizes. It should be noted that bands for 38mm and 40mm devices are compatible with each other, as are straps for 42mm and 44mm iwatch strap
To install a new watch strap, you need to follow these steps:
-To commence, find the band release button on the rear of your Apple Watch, near where the strap is attached.
-While holding down the strap release button, slide the strap sideways in either direction.
-The strap should easily slide out of the Apple Watch slot.
-The Apple Watch should now have one end of the band removed.