These apple and cinnamon muffins are best when you let the muffins cool completely before adding the glaze and then letting the glaze set before storing them for good. You can place them in an airtight container or big enough bag for a few days and keep them at room temperature. I don't recommend refrigerating them or they'll dry out faster. Try not to squish them in the container or bag, and if possible, store in a single layer. Use parchment paper between layers if you must layer them. They can be frozen. Wrap to protect against freezer burn for up to 3 months.