Changing an Unfiltered Fish bowl for a Betta

If i have a unfiltered fish bowl and I am going to change the water how much do i change? 20% or 50%? Do I take out the water that I'm putting back into the bowl and then dump the rest? And I know possibly dumb question, but what exactly is room temperature in the water? And also how would I get the water temperature to 75 degrees?

well it all depends on the size of the bowl im going to say a gallon bowl? i would change the water 50% everyday or two....well when i change my betta bucket things which are only like a gallon or less because i dont have room for 10+ adult males i just put the betta in a cup with the old water and completely dump out the bowl thing and let the new water sit for a while treated and then put the betta in with the little bit of old water and there always fine after that