How to adjust output temperature of a PTC ceramic heating element?
The whole point of a PTC heating element is that it's PTC, Positive Temperature Coefficient.

As the element heats up, the resistance increases, usually sharply at the target temperature, reducing the current and so heat generated, when operated from a constant voltage supply. It automatically controls itself to its inbuilt transition temperature. As a result, these make very safe heaters, as if the heatsink fails, they stop dissipating automatically. The particular one you linked to specifies 180+/-10, though it's not clear whether that's C or F.

There are two main applications for which you would specify a PTC heater. One, where you want it to automatically control its own temperature at its transition temperature in normal service. Two, where you are controlling the temperature below that, but want a self-acting temperature limit should the controller fail.

You could control the temperature below the transition temperature by using a temperature sensor to feedback and limit the drive voltage. If you knew the resistance/temperature curve of the device, either from a specification or from measurement, you could use that as the sensor, with a little complication in multiplexing it for heat or sense duty.