You should replace just over 1/3 of the water, enough to get the CYA level under 100. Since the tests that are able to measure CYA levels over 100 aren't all that accurate it might be best to do a little extra just to be sure. You can do the water replacement various ways, partial drain and refill or continuously adding and removing water at the same time. Unless you have a very high water table it is generally safe to lower the water level about 18" below the skimmer.
If the water is clear and you bring the FC level up to 12 or higher the pool will be fine to swim in for now. Add five gallons of 6% bleach and let it circulate for an hour and that should take care of things for today. As long as the water remains clear and you keep the FC at 12 or higher at all times (aim for perhaps 15, so that it doesn't go under 12) you can limp along with high CYA for a little while.
If the water isn't clear don't even think about swimming.