I buy API Aquarium Salt, however some people do use their own salt (ask them though, not me about that).
There will be good directions for you to follow on the box. Sorry, can't think of it right now, hah, but the directions are straight forward. If you're going to use at-home salt, don't ask me, lol.
Change for both. If you take out 50%, put back in 50% salt. If you take out 100%, put in 100% (just make SURE it's ABSOLUTELY 100% water taken out, if it builds up, it won't be good for your betta).
If you see your water line lower, or you see your water evaporate, don't replace salt. Salt does not evaporate. Only if you manually remove it, then you manually replace it. Hope it helps :)