The punishment for apostasy is left to the ruler of the country who may or may not punish the apostate depending on how serious was his/her crimes surrounding him/her leaving the faith. Like during the lifetime of the Prophet there were some men who claimed to become Muslim and asked our Prophet for help. They were sick and so he sent them with a camel herder to help cure them. Then these men apostated,killed the camel herder and also stole the camels so they were when caught put to death for murder and armed robbery along with apostating.This was then taken to mean that all apostates who leave the faith are to be killed which is not the case.

On the other hand there were men who commited acts of apostacy and who the Prophet didn't punish. Of them one came back to been a Muslim and was forgiven and accepted back as a Muslim and the other died a non Muslim. 

Today we see some rulers put their subjects to death for apostacy and some don't. As individuals we cannot kill anyone who leaves the faith nor punish them. This is what I know and God knows best.