Which label are you talking about?
In a chart you can rename the title of an axis (rightclick the axis => edit axis => titles)
Or you can rename the tooltip when you hover over a point (worksheet => tooltip...)
Ah good point that one - Don't know.
I usually just rename the column or if that is making a hash somewhere else, I duplicate the field and name it what I want to show.
showing same dimension with different names is easy as we can duplicate the same dimension with many names. However showing multiple dimensions with same name on charts is a pain. Almost all BI tools give us this capability to just give a label to the dimension, and the underlying dimension name stays the same. This way, we can use same label for different dimensions in a chart. When you rename the label of a dimension, it does not change the name of the dimension. It should only change the display name of the dimension.
In the above image, I thought there was an easy way to do this. However for some reason, tableau does not allow us to do this