Not sure what you are doing wrong, because this is pretty easy to do.
Select the style and variation you want to edit, select style creator, delete the channel you want to replace and re -record it. Save the style as a new name on the user or USB drive.
Best way to check it is working is to select the style you edited, ( make sure it is the new one not the original which will still be there if it is a default style) and then go into the mixer and lower the volume on every channel except the one you edited and play the style to see if it is working the way you want.
If you are doing all this but it is not working check a few things - is there something within the parameters of the current style that limits the range of the instrument. Easiest way to do that is to try doing it on another style and if it works, you then know that it is something specific within your target style and you have to find it.
Mike