Fixing a doll's hair is remarkably similar to fixing anyone else's hair. As long as the doll's hair is long enough, you can easily make an updo or bun in the doll's hair and the technique is the same as the technique for working with a human's hair.
With a few elastic hair bands and bobby pins you can easily make an updo or bun for a doll that looks great.
Brush through the doll's hair so that it is tangle-free.
Pull the doll's hair back into a tight pony tail and secure it with an elastic band. The height of the pony tail will be the point of the bun or updo. If you want a low bun make the pony tail low. If you want a high bun make the pony tail higher.
Take the pony tail and begin twisting it around in a spiral to create the bun shape.
Secure the bun with either bobby pins or a second elastic hair band when all of the pony tail has been wound up in a spiral shape. The bobby pins should be inserted around the outside of the spiral shape and pushed in to the center of the bun. If using an elastic band simply place it around the spiral shaped bun to secure it.
Pull a few random tendrils of hair from the bun for an updo style. These tendrils are supposed to look random and somewhat sloppy. You can secure them with more bobby pins if necessary.