Before we consider trimming it, let's explore a few troubleshooting steps to see if there's an alternative solution.
Firstly, make sure you have properly adjusted the normal band of your Apple Watch. It's possible that there is some extra slack in the band that you can tighten, which might eliminate the need for the extender. Refer to Apple's support article on how to adjust your Apple Watch band for step-by-step instructions. (see below)
If adjusting the normal band doesn't solve the issue, you could try folding the extender over itself to create a double layer of the band. This should effectively reduce its length without needing to trim it. Give this a shot and see if it provides a more suitable fit for your needs.
If none of these options work for you and you're still determined to trim the band extender, I would advise caution. Trimming the extender may void any warranty or return policies, so proceed at your own risk. If you decide to go ahead, use sharp scissors or a cutting tool to carefully trim the extender. Remember to trim a small amount at a time and test the fit frequently to avoid cutting off too much.
Happy diving!
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