How can I get iOS 15.1?
Update version 15.1
[Re-Titled by Moderator]
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What does it say for iPhone Model and Software Version?
You need an iPhone 6s/6s Plus or newer in order to run iOS 15..
Supported iPhone models - Apple Support
If your iPhone is an iPhone 6, then you cannot upgrade beyond iOS 12
have you read thru this support article for updating?
Update your iPhone, iPad, or iPod touch - Apple Support
need more storage for update?
If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support
issues?
Updating iOS: common issues and solutions - Apple Community
video..