Same as they are born. Through enough faith and worship.

Well, actually we never get to see a god being reborn due to the faith in it reappearing - but in theory it is entirely possible a forgotten god gets reborn due to its religion or belief rising up again (though it needs to be a real faith and belief).

What we do see however is rebirth through active worship. A god who is dead can be resurrected if acts of worship great enough are performed - such as a sacrifice powerful enough to give them back their energy.

However, to do so the god must still be known. We actually see in the afterlife of the old gods that they are divided in two - those that simply do not receive faith and worship anymore ; and those whose very name was forgotten. If someone forgot a god (as in their name, their religion, their myths and people are forgotten by everyone), it means they cannot come back.

For a god, death is not so much a trouble because there is always a possibility of coming back thanks to worship or sacrifice. Being forgotten however, now that removes the possibilities of resurrection.

Ok if a god is reborn he comes back with the same memories the same personality and if a god is killed what happens to him like zoriah