need info

I purchased a complete upper from psa.The handguard was a lot fatter than i wanted.Can i just replace it with a thinner one or is there some reason the fat one is needed for some particular reason?As you can see I don't know very much about ar's.

Replace it with a more comfortable one, as long as it has good heat shielding. The width ("fatness") of the handguard isn't an issue but the heat shielding is.

Edit: I recommend the MOE handguard from Magpul: http://store.magpul.com/product/MAG440/184

PSA allows the option of a mapul handguard when buying an upper. Maybe they will trade it out for you? It cannot hurt to check, just realize the magpul handguard costs more.