Window Won't Stay Up

I purchased Reliabilt 5600 series double hung replacement windows and had them installed by Lowe's in 2008. One of them will not stay open. I am not sure how to remove or replace the part which I believe has failed. I believe it's the constant force coil balance. I need to know how to remove and replace the part and which parts I need for this window. Thanks so much for your assistance. We use this window all the time and it is important to us that it works the way it should. Thanks!