Due to my dr. he want 2 modify my combo as :reduce prozac from 40 to 20 mg and increase wellbutrin (bupropion) xl from 150 to 300 mg,how can i apply that by safe?

Should be okay: That change should be safe to implement simultaneously, you may want to talk to your doctor about taking the lower dose of fluoxetine for a week and then taking the higher dose of the bupropion extended release although that is a just a precautionary move and largely unnecessary for many patients. Stay in close touch with the office of the prescribing physician during the transition! Stay safe!