How do I get iOS 15

Go to Settings->General->Software Update and follow the onscreen prompts. Link-> Update your iPhone, iPad, or iPod touch - Apple Support

If it issues an error or another message, please post back what it says exactly.

In addition to what Phil0124 said, if your iPhone is an iPhone 6 or earlier you won’t be able to get iOS 15 as the hardware doesn’t meet thee requirements for the newer versions of iOS.