Never buy betta fish. Always wait until the final release fish.
Trial over?
Full water change = max stress.
Should only be done in an emergency.
To reduce shock/stress you must make sure the temperature of the old and replacement water are identical. Use a thermometer. Touch isn't good enough.

pH should also be as realistically close as possible. Ditto for general hardness (GH).
Most importantly if using water from your tap and you're on a municipal water system you MUST absolutely use a product that removes chlorine/chloramines (ex "Stress Coat", "Amquel") and has additives that help prevent loss of the fishe's protective (slime) coat.
Bettas are fairly hardy but a sharp change in pH and using unconditioned makeup water that's been sterilized with chlorine is a certain death sentence. They have a short life expectancy to begin with and keeping them in a bowl with no biological filter is rather harsh.