Diagnosis
The internal thermostat has failed in the "on" position, causing the iron to continuously heat to its maximum temperature regardless of the dial setting.
How to Fix It
Unplug the iron immediately. This is a critical safety fault. Do not use it again.
If That Didn't Work
There is no alternative fix. A failed thermostat is a definitive end-of-life for the unit. You must replace it.
Heads Up
An overheating iron is a serious fire risk. Never leave it plugged in unattended, and do not attempt to repair the sealed thermostat unit yourself.