Yes, you can heat up the sake bottle in hot water. However, please do not heat products with gold or silver in the microwave as sparks will fly.
How to heat up a bottle of sake
1. Put the sake bottle in the pot and pour water into the pot until the bottle is half full.
2. Remove the sake bottle and heat the water in the pot.
3. While the water is heating, pour the sake into the bottle.
4. wrap the spout of the sake bottle.
5. When the water in the pot comes to a boil, turn off the heat.
6. Put the sake bottle in the center of the pot to warm it up.
7. Pour the warmed sake into a sake cup and drink.