What is the best and safest way for my baby to lose weight?

Focus on activity. Babies tend to loose weight quicker when they grow - so focus on activity and boost their metabolism.
NOT !!! Your baby should not lose weight! even if your baby is "overweight" you should rely on the growth process for the baby to become more weight appropriate. After all, unlike adults, babies grow. So the key is healthy diet and activity for your baby to become a more appropriate weight. Add fruits and vegetables and moderate the more calorie-dense typically carbohydrate-heavy foods.