Can I update my iPhone from 12.1 to the 15.1

If you're still on 12.1 you most likely have an iPhone 5S or 6 which can't be updated beyond 12.5.5.

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.5

• If you have an iPhone 6S through 13, you can update to 15.3.1

Go to Settings > General > Software Update.

Update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone, iPad, or iPod touch - Apple Support

Update using your computer -

Update iOS on iPhone - Apple Support

If it's an iPhone 6s or newer, it can be updated directly to 15.3.1. Otherwise, no.

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