Probably the fan is clogged with dust, quite likely matted behind the air exit vent. Poke a pin in there to try and break up the mat then blow in there, with an air duster if you have one, and suck on the air inlet with a vacuum cleaner.
If that doesn’t work you’ll probably need to disassemble it to properly get at the fan. There’s a disassembly video at https://www.youtube.com/watch?v=d21J2ddoQPE
The tool he’s using to crack open the case is a jimmy, available from the iFixit store - very useful.
You’ll find Speedfan (www.almico.com/speedfan.php ) useful for monitoring the internal temperatures.