A 42-year-old woman with bipolar disorder has been on olanzapine 15 mg/day for 14 months and is psychiatrically stable. She presents with a 6-month history of progressive weight gain (12 kg), fasting blood glucose 118 mg/dL, and triglycerides 280 mg/dL. Her BMI has increased from 24 to 29 kg/m². She is very concerned about her appearance and metabolic changes. What is the most appropriate next step in management?