There are many things which can cause a system to overheat. The major ones are as follow for a Windows operating system:
1) When the cooling fan has accumulated debris or the vents are blocked.
2) The heat sink is faulty thereby losing the ability to dissipate heat properly from the processor.
3) Thermal compound on the processor has completely dried up and can no longer carry out its function.
4) Running high specifications-consuming applications on a low system hardware.
5) Some malicious software using up the memory and more part of the system storage can be a culprit.
To find solution to your system you would have to tell us more about the conditions of the system.
Does the system whiz heavily at some point in time? What application do you usually run that makes the temperature to go up? What is your system configuration?
At this point in time you can try to prevent the vents from anything that can block them. Place your system on a flat table or desk when using it. Listen for changes to the noise of the cooling fan and use compress air to open block vents. Scan your system for malware and disable many applications running at the background at the 'System configuration'.
If you are good with hardware coupling and fixing you can open up the system and wipe the previous thermal paste off the surface of the processor and the heat sink and reapply new one.