Man, I've been in that same situation before, young plants in hydro veg can be finicky. I set my ph at 5.8 and it usually rises over a couple days although 6.2 doesn't sound horrible, sometimes mine climbs to 6.5, depending on setup and strain. Once my plants get older they usually look better. As far as ppm goes I would chng res water, add light nute solution, maybe 30% or 40% of directions, watch ph. Less is more, especially with young plants, some strains need to be babied into a high ppm. Once plants are bigger and have more foliage, hack the old yellow leaves. Remember signs of improvement show in the new growth, not so much in leaves that have already started to fade. That's my 2 cents, hope it's worth something to ya. I always stress over my little babies but in the end, they always pull through. Keep a positive state of mind and try to relax.
Best of luck.