I want to update to 15.7.3

Update your iPhone, iPad, or iPod touch

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.7

• If you have an iPhone 6S through 7, you can update to 15.7.3

• If you have an iPhone 8 or higher, you can update to 16.3.1

Go to Settings > General > Software Update.

This link will show you how to update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone or iPad - Apple Support

This link will show you how to update using your computer -

Update iOS on iPhone - Apple Support

falanwo Said:

"I want to update iOS to 15.7.3"

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Okay. So, what's the model of your iPhone? You many not be able to, depending on your model.

Updating an iPhone:

Update Using a Computer:

You can use a Computer to update. Read My User Tip: Making Use of a Computer for Updating an iOS Device - User Tip. If any updates are available, they will install, sure enough.