How does a bone marrow transplant treat sickle cell disease?

A bone marrow transplant is used to replace defective and ineffective bone marrow with healthy and effective marrow.  

Explanation:

Red blood cells are manufactured in the bone marrow.  The bone marrow of a sickle cell patient produces red blood cells that are sickle shaped and ineffective.   Chemotherapy is used to kill all of the defective bone marrow and a bone marrow transplant is used to replace it.

you should seek treatment when you are seriously thinking of sui.cide, and that can eventually lead to your own death.