I need to update my iphone6
How to update my IOS 12.5.3 to 13
iPhone 6, iOS 12
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you need to free up some storage to install it
read
If you need more space for an update - Apple Support
otherwise you can try to connect it by cable to a computer and update it that way don't believe that require as much free storage
you can read about the procedure here
Update your iPhone, iPad, or iPod touch - Apple Support