1 Cor 15:51 says “Behold! I tell you a mystery. We shall not all sleep, but we shall all be changed”.
The idea of sleep is used by the NT writers to describe the death of believers. Just in the same way that when you go to sleep, you are unconscious but when you wake up you are conscious again, when you die you are unconscious but when you are resurrected you will be conscious again. So when Paul says “we shall not all sleep”, he means that not all believers will die (i.e. there will be some believers alive when Christ returns).
Paul says that those resurrected will have a different body. He says “what is sown is perishable; what is raised is imperishable” (1 Cor 15:42). Those who do not die will not be resurrected but will receive this new imperishable body. We cannot enter the Kingdom in our present bodies (1 Cor 15:50), we must first be made imperishable (1 Cor 15:53). That is why he says “he shall all be changed” (i.e. all those entering the Kingdom will have a new imperishable body).