Can you heat up bottled water in the microwave to put in your betta's tank?

Yes. I can't find anything wrong with using a microwave to slightly heat up cold water for fish, esp. tropical (warm-water) fish like Bettas. Just make sure to check the temperature with a thermometer and stick to the ideal 70-80-degree range. I'm less sure about heating "treated" water in a microwave (which can contain many varieties of conditioners, medications, etc.), but at the very least treat the water many hours in advance.