A 74-year-old woman with non-valvular atrial fibrillation, hypertension and type 2 diabetes (no prior stroke or TIA) attends for review. Her CHA2DS2-VASc score is 4. She is currently taking metoprolol and aspirin 100 mg daily. Her renal function is normal. According to current best practice, what is the most appropriate change in her antithrombotic therapy?