Due to a recent study, it is said that people who were victims of child abuse as a child have less grey matter in their brains. how do i increase it?

Don't need to.....: You don't need to increase the amount of grey matter - you need to optimize what you have there. Most adults are only utilizing 5-8% of their brain cells, so we have almost unlimited capacity to utilize more of what we have. You don't need more numbers of cells. You need to recruit the ones you're not using to work. This is done by brain exercise. Read, listen to music, juggle, do crosswords......

Many ways: Science is now appreciating that there are many ways to enhance brain structure and function. Among the most well documented are mindful meditation, exercise, Yoga, a healthy diet, sufficient sleep, and a sense of gratitude. Learning new things, listening to music, social connectedness, prayer and hypnosis are also good candidates.