Your questions is hard to answer. THC is non-toxic to humans. That means that no amount consumed has the potential to do you any real harm. It may have side effects, but will not kill.
So in effect,...
You can't. You either have low-grade marijuana, mid-grade marijuana, or high-grade marijuana. There is no magical alchemy that can change schwag to skunk. If you want better weed, then you have to...