What should I do if I have a defective hip replacement?

See your surgeon. Despite what you might hear in the news, many hip replacements that have been recalled are only defective in a small number of patients. Your surgeon can monitor you hip with regular x-rays and possibly blood tests.
Identify problem 1st. A "defective" hip replacement needs to be carefully defined as to what is determined to be "defective". It is very unusual for a total hip replacement to be truly "defective", despite what the advertisements on tv might suggest. Check with an orthopaedic surgeon who specializes in hip replacement surgery for an accurate assessment first. He/she can then provide guidance to you.