You should be able to see the thresholds under
/sys/devices/platform/coretemp.?/hwmon/hwmon?. The values replacing the question marks will depend on your system; the first is likely to be 0.
You’ll see a number of files named
temp?_max. The values replacing the question marks vary again, starting at 1, and increasing to cover temperatures for the package and all the (real) cores in your system.
_label tells you what the set of values describes,
_input shows the current temperature,
_max the maximum temperature,
_crit the critical temperature, and
_crit_alarm indicates whether the critical temperature alarm has triggered.
You can’t change these using kernel-provided interfaces, they’re set by the CPU and/or system firmware. You shouldn’t try to change them anyway, they’re designed to protect the CPU from damage. If you’re bothered by the overheating messages you should try fixing the overheating problems instead — the ambient temperature might be too high, the CPU’s cooling could be hampered by dust, etc.
The messages in your kernel’s logs don’t reflect the kernel’s decisions; they’re the translation of machine events that the kernel is being told about. Your computer noticed that its CPU was getting too hot, so it throttled it, on its own, and told the kernel about it.