At this point it might already be too late, but...if you really want to help it, go to a qualified Vet in your area. Then look up care of red ear sliders...there are loads of sites that will give you good care help. You need to keep the temp. in the tank high about 88 or so degrees, more water, a heat lamp on the basking spot...all these things would be temporary until she (or he) can get better. Then you would need the proper set up...
Unfortunately at this age they are very delicate and really need proper care. But I think it is wonderful that you are trying to help it. Good luck.