I want to update my iPhone

An iPhone 6 can't be updated beyond 12.5.7, it doesn't have the hardware needed to run anything higher.

If you need a higher iOS version, you will have to purchase a newer phone.

To update to 12.5.7 _

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

ishanatanwar8765 Said:

"I want to update my iPhone"

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Updating an iPhone:

Use a Computer to Update:

Use my User Tip:  Update your iPhone Using a Computer - User Tip. If any updates are available, they'll install, sure enough.