The short answer is horribly. But okay, you might want more details than that.
He was discovered in 1922. Since then there have been numerous ideas on how he died. Some have been pretty odd.
X-rays conducted in the 1960s revealed a fragment of bone in his brain cavity. It was possible evidence of death by a bonk on the head. But closer study showed that the sliver probably broke off after death.
In 2005, a broken leg and massive chest wounds led to the conclusion that not only had he fallen off a chariot while hunting but he also was run over by one.
DNA tests from 2010 pointed to malaria. The illness would have made his broken bones even worse.
In 2013, a doctor examined CT scans. He stated that the chest wounds actually came from the bite of a hippopotamus. I swear I can't make this up.
So here's my scenario. The unfortunate boy-king gets clocked in the head by a jealous rival. It happens while he is out hunting hippos by chariot. He tumbles off and is run over.
The hippo charges. Tut manages to get up and run. But his clubfoot, broken leg and malaria slow him down. The hippo practically bites him in half. Then Tut spontaneously combusts.
Yes, the most recent idea is that Tut spontaneously caught fire in his coffin.