I want to change my iPhone iOS

To update your iPhone, select Settings > General > Software Update. If your phone is eligible for an update, you will be prompted to do so. You can read more about it in the link below. If you have any problems doing this, let us know. Please include any error messages and tell us which iPhone you have and what its current iOS is.

Update your iPhone or iPad - Apple Support

Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information.