My Apple Watch will update due to not enough storage.
I have a Apple Watch Series 3. I am unable to update due to not enough storage.
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Community User
Unpair your Apple Watch via the Watch app on your iPhone.
Then Re-pair your Apple Watch,this frees up a surprisingly large amount of storage space.And without any loss of your data.
Or this;
If you don’t have enough space to update your Apple Watch – Apple Support