Rebirth is used in Elden Ring to Respec your Character Attribute Points. You have to unlock the rebirth option at NPC Rennala, Queen of the full moon. You can change your character build with birth depending on your choice. For example, you want a strength build so you can do rebirth to respect your Strength Attribute points and make yourself a Strength build.
Rebirth at Rennala, Queen of the Full moon
For the Rebirth, you need to go to the Raya Lucaria Academy in the Liernia of the Lake region of the map. To make your way inside the Raya Lucaria Academy you need a special key called Glintstone Key.
After you’ve gotten the Glintsword key and unlocked the Raya Lucaria Academy and entered the Academy, You need to go to the Debate parlor. You will have to fight a boss here and then further make your way to the Raya Lucaria Library. There, you will fight the second main boss of the game, Rennala, the Queen of the full moon.
After you’ve defeated Rennala, Queen of the full moon. You can simply go to the Raya Lucarian library to talk to her and choose Rebirth from the Option which will be used to respec your Attribute points.
For Rebirth to work, Rennala requires a special items called Larval Tears. Here, are the locations to find the Larval Tears so you can continue with the Rebirth. It is required by Rennala, one Larval Tear for the Rebirth to work and respec your Attribute points.
Village of the Albinaurics
For the First location you need to go to the village of the Albinaurics, when you’ve reached there you need to keep on moving and when you reach the tombstones. There will be a dead body lying behind the tombstone. You need to go to the dead body and pick up the item and you’ll receive the Larval Tear.
Abondend Merchant in Siofra River well
For the next location of Larval Tear, you have to go to the Siofra River well at the given location on the map.
Once, you have reached the Siofra River well you need to give in the well take the elevator down and you need to keep on moving until you reach the Worshipper’s Wood. You will now go into the Worshipper’s wood where you’ll meet the NPC merchant called Abandoned Merchant. You have to go talk to the merchant and you can purchase the Larval Tear from the merchant’s shop.
Royal Moongazing Grounds
For the next location for the Larval Tears you need to go the Royal Moongazing Grounds by making your way by the road to the Manor. You have to keep moving from the Royal Moongazing Grounds and you just have to go straight and you’ll reach the open plains called Three Sisters. You will see there is a castle there you need to make your way into the castle and you’ll find a NPC in the castle.
The NPC Pidia, Carian Servant, you can talk to her and you can purchase the Larval Tear from her for 3000 Runes.