There is an easier way:
DS.Properties.VarNames{3} = 'NewName';
DS.Properties holds the array's metadata: variable and observation names, variable units and descriptions, and a description for the array itself, among others. You can see all that by typing
DS.Properties
BTW, there are two problems with the code as you've written it:
1) 'new name' isn't a valid MATLAB identifier, and so not a valid dataset variable name, and 2) calling set without assigning to something has no effect -- dataset arrays are not handles. You'd need to do this:
DS = set(DS,'VarNames',vn);
which is somewhat non-intuitive, and so you're better off accessing the metadata directly through .Properties.
Hope this helps.
