If your child is receiving 504 services, but you do not believe their behavioral supports are effective, you should consider requesting an evaluation for special education services. See form 3 on page 74 of the IDEA manual for a sample letter. In your letter you may leave out the request that your child be tested for a 504 plan if your child is already receiving 504 services.
If you do not believe that your child needs special education services, consider calling a 504 meeting and request an Functional Behavior Assessment (FBA). See here for a sample form on requesting an FBA.