Make Brown Sugar Peach Simple Syrup: Combine frozen peaches, brown sugar, and water in a medium saucepan over medium-low heat. Simmer, stirring occasionally until brown sugar has dissolved and mixture has slightly thickened; about 5 minutes. Cook 2 minutes more. Strain out peaches. Pour simple syrup into a glass jar with a tight-fitting lid. Store in the refrigerator for up to 2 weeks.