Super easy. The only potential difficulty is finding a thin enough 17mm wrench to hold the p-stat while tightening up the fitting (Also 17mm but width doesn't matter on the fitting)

It will often be on the end of a copper tube that started at the top of the boiler, travels down and then back up to try and keep water away from the valve. It might look different but should have 2 or 3 wires connected to it. You might take some pictures of the things on your machine with wires attached so we can point and say, that's the one.

What's the square thing at the very top of the second picture. It has a pipe like I described that goes down and then up. It looks like a Sarai pressurestat. Those don't usually fail though the can be rebuilt or the connecting tube could be clogged. Pretty easy to take it off and check. Also, in the future it's a lot better if you post the pictures here so they don't vanish in the future. I'm always wary of clinking links that take me off this site.

The Sirai is typically considered a much better piece than the Mater, but there should be no reason you can't use it instead if the wiring supports only one switch. The Sirai in my machine can switch 3 phases but the Mater can switch only one. And you' don't ream pipes like that, you descale by letting them soak in a vinegar or citric acid solution. I always try to support them so the bubbles only come out one end so they act as a pump to move the solution through the pipe.