If you need to rename a field in a single visualization, you can simply drag the field into the visualization and then click on rename.
But what if you want to rename a field in several visualizations but not all of them. There is a work around that can get you to this. You need to create a new measure with a simple formula:
[New Name] = [Measure that you want to rename]
This can then be applied to any visualization that you need to use the alternative name.