Could someone please explain how this works?

Greetings All,
This link:
http://www.vellemanusa.com/downloads/0/manual_mk138.pdf
is the schematic for a thermostat kit I built. Even though it works I
don't understand how it works. The high temperature limit is too low
for what I want to use it for. But I think that using less resistance
for R5 will raise the upper temperature limit. Am I right? And if I
am, why?
Thank You,
Eric
.