What are my steps to get the original custody order between my ex wife and I changed?

After our divorce, I was granted custody every weekend. During this time I was suspicious of my ex wife and her husband abusing drugs. In August 2016, they were eventually both found with drugs. Her ex husband on an overdose in their home while my children were there and her while driving with drugs in her system with my kids in the car. DHS pulled my children from their mothers house and placed them with me at that time. Now I want to get the original order changed so that they’re with me. The courts and DHS goal is to eventually have them placed back with their mother. Thanks