Is it an abbey from china or indonesia?
There are two ways I know of, both are pretty much the same after the first step. You'll need to warm up your doll's head/neck, some people choose to do with with warm water while others have used hair dryers or other such devices, personally I sit mine infront of the vent of my space heater for a few seconds (I read somewhere that a young lady lived in an area so warm all she had to do was leave the doll out in the sun for a period of time, so basically whichever you're most comfortable with).
A tutorial with pictures of someone removing the doll's head/hair via warm water method:
As I said, it's all pretty much the same thing after you find a way to heat up your doll's head. Just cautiously wiggle and work the doll's head off, and be careful not to snap the neck peg or the anchors.
I suggest practicing on some not very expensive dolls before trying anything out on Abbey. c: