Since your tank isn't filtered, the toxins that build up in your water will cause you to do a 100% water change every other day, along with a 50% on the days you don't do 100% water changes. Yes, doing 100% may put your betta under stress but that's the reality of having a 2 gallon heated tank. But there is an easier way, get a 5+ filtered gallon tank and cycle it for a few weeks, then put your betta in there and do a 1 gallon water change every week. By cycling the tank, there would be beneficial bacteria to handle your betta's waste and reduce the quantity and frequency of water changes. Here's a list to changing the water of your 2 gallon unfiltered tank.
1. Put 1 or 2 gallons of water in a bucket (or other water holding device) and add water conditioner. The tap water conditioner will get rid of any chlorine or chloramine that your city puts in your water that makes humans safe to drink it, but this is toxic to fish. Leaving the water out uncovered for a few days gets rid of chlorine, but it won't get rid of chloramine, so the conditioner is the safer choice. Prime is a pretty good water conditioner for its price.
2. Remove water from tank. Preferably remove the water with a gravel vacuum. Using a cup and somehow taking out the poop and leftover food might work, but a gravel vacuum is much better if you have gravel so you can take out the debris stuck in the gravel.
3. If your water has been left out w/ the conditioner for at least 5+ minutes, slowly add water to tank. If you did a 100% water change, put your betta in a cup beforehand and add.
Variations to these steps may be made to fit your lifestyle, but always do 100% water changes and 50% the next day and so on.