My 6yr old lost some of her baby teeth and when they started coming out again some are coming out at the back of the other.what can i do?

This is not unusual: Your baby's jaws are growing but the teeth coming in are actual adult size. In time, those teeth growing in behind will move forward displacing the baby teeth as your child jaws grow in diameter. Nothing needs to be done at this time.

See your dentist: Sometimes childrens teeth are crowded and they try to come in behind. Some other baby teeth may have to be removed to allow first few permanent teeth to come in correctly.

Lower Front?: If the area of concern is the lower front, then that is normal. The lower front permanent teeth start coming in behind the "baby" teeth. Many parents panic when they see this occur however when the baby teeth are gone then you can see the permanent teeth in place. At that time a determination can be made if there is sufficient space for these teeth.