How do I abolish sin in Dark Souls 3

Solution 1:

In the Undead Settlement, locate the Morticians Ashes in the graveyard where the giant shoots at you. Take these back to the Shrine Handmaiden, and you can purchase the grave key from her. Buy this key, then go to the Dilapidated Bridge Bonfire in the Undead Settlement, and head right into the sewer with the rats. On the wall opposite from where you enter will be a door, use the Grave Key on the door to open it. Head down the corridor, then right.

The statue of Velka will allow you to absolve your sins at the cost of souls.

Solution 2:

In the Undead Settlement, you will find a statue of Velka, the Goddess of Sin. She provides Absolution Services at the price of souls (the amount is determined by the amount of sins committed).