IMPORTANT: She will not change your face if you are a vampire. Also you dont have to be a member of the thieves guild, and it will cost 1000G
To find Galathil (the face changer) you need to get to the Ragged Flagon if you know where it is skip to the last paragraph. First go to Riften via carage or quick travel, then go down the wooden planks by the water you should find a door leading to The Ratway proggressing through this is linear and filled with bandits and lowlifes that may be dangerous depending on your level. After the first two attackers are dead there will be a drop off if you can pick the expert lock on the gate you can skip right to the end of the ratway. Otherwise follow the linear path and at the end you will enter The Ragged Flagon.
Once inside you will find a High Elf (Galathil) sitting in a chair, on the porch by the water. It will cost 1000 gold to change your face and the achievement will pop before you even make any alterations. (In case you want to know she can edit weight, skin tone, and all other facial features but not race or gender)
NEW: I recently found out you can cure yourself when your a Vampire Lord by the traditional method "Give the Jarls Mage in Morthal a filled black soul Gem" and Re-Lord yourself by talking to the Daughter of Coldharbor (The main character in the Dawngaurd DLC)
BUT BE VERY CAREFUL. On my data she disapeared completely and after checking on the Skyrim wiki this is a known glitch.