How can I install the new update

If you're still on version 12, then you probably have an iPhone 6, not a 6S.

• 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

oby129 Said:

"How can I install the new update: How to install iOS 15.3.1"

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Update Using a Computer:

Use my User Tip:  Update your iPhone Using a Computer - User Tip

Back Up First:

IMPORTANT: If you have an option to update, however, then be certain you backup your iPhone first. That way, you will have something to restore your iPhone from should anything go wrong with the update.