Officially, he died of natural causes (brain haemorrhage), an unpleasant death, but at least in his own bed.
Although it is impossible to be certain, I think it unlikely that Beria’s later claim to having murdered him has much validity.
It is possible that a doctor, concerned about Stalin’s latest purge (on Jewish doctors) may have used warfarin on him, but that too is impossible to verify. In the absence of any evidence to the contrary, I’ll go along with the official verdict as most likely - especially in the light of Stalin’s excessive consumption of vodka over many years.
I suspect, we will never know for sure.