CV = changefieldf(V, 'var1', value1, 'var2', value2, ...) returns a 
PIVMat structure array CV identical to the input structure V, in which 
the variables 'var1', 'var2' etc. receive the new specified values 
value1, value2 etc. Variables in a PIVMat structures are labels and 
units of x and y axis, values of x and y coordinates, etc. See loadvec 
for a complete list of available variables. 
 
V = changefieldf(V, 'unitx', 'm') is equivalent to V.unitx = 'm'. 
However, if V is a structure array of vector/scalar fields, changefieldf 
applies the changes to each element V(i). 
 
For instance, PIVMat structures have their axis labels named 'x' 
and 'y' by default. In order to change these labels to 'X' and 'Z', 
enter the following: 
 
   v = changefieldf(v,'namex','X','namey','Z'); 
 
You can also specify non-string arguments for the value1, value2... 
For instance, in order to invert the values of the x axis, 
 
   v = changefieldf(v, 'x', v(1).x(end:-1:1)); 
 

shiftf, extractf, resizef, operf. 
Published output in the Help browser 
   showdemo changefieldf