If you're referring to a texture's sharpness, going into the attribute editor and turning the texture file's filter type to "off" should do it. You could also up the resolution in extra attributes to to match your texture resolution but I'm not sure offhand if that will affect anything. If it's just a general render quality issue, make sure you're choosing the appropriate render setting in render globals (e.g, set it to Production quality and customize from there).
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