add fresh water to the res, if u have it full take some of the nutes out, such as a gal of nutes replace with ph adjusted water, it will lower the PPM quite a bit... since u caught nute burn early, it should not be an issue, IF you see the yellowing on the outer leaves progress after 3 days to a week, do a complete flush and a flushing agent to remove the salt build up and then start with a 3/4 solution.
good luck
-cody