How can I get my legal guardianship changed?

I have major family issues, and my current family is causing me major depression which is lowering my grades and causing me to be socially awkward, depressed, and I fear if this continues much longer I may hurt myself. I have a step brother who is legally an adult, as well as his Fianc�e, who I am very close with and they treat me like I am their child. They help me when I'm down and treat me better than my current guardians. I want to change my guardianship to them, they are willing to take me in and are financially stable (more financially stable then my parents) and my counselor also agrees it would be best for me. So how do I get my guardianship changed?