With the release of the Apple Watch Series 7, Apple Watch comes in 41mm and 45mm sizes. The Series 4, Series 5, Series 6, and Series SE come in 40mm and 44mm models, the first three generations of Apple Watch( Series 1, Series 2, and Series 3) are available in 38mm and 42mm sizes.
But the Apple Watch band sizes are different, there are two kinds of Apple Watch bands:
38mm, 40mm, and 41mm
42mm, 44mm, and 45mm
If you are unsure of the size and model, just check the backside of the Apple Watch.
How to remove your old Apple Watch bands
1. Locate the band release button on the back of the Apple Watch, near the band connection.
2. Hold down the band release button with your fingertip or fingernail, slide the band sideways in either direction.
4. Repeat the same process for the other end of the band.
How to install the new band
1. Slide the new band into a groove in the Apple Watch and you will feel and hear a soft click.
2. If you don’t feel or hear a click, just slide the end of the band side to side to see if it is securely fastened.
3. Repeat the previous step until both ends of the new band have been installed.
Note: The band release button doesn’t need to be held for this process.