legacy/src/main/java/ucar/nc2/NetcdfFileWriteable.java
public void updateAttribute(ucar.nc2.Variable v2, Attribute att) throws IOException {
    if (defineMode)
      throw new UnsupportedOperationException("in define mode");
    spiw.updateAttribute(v2, att);
  }
public void updateAttribute(ucar.nc2.Variable v2, Attribute att) throws IOException {
    if (defineMode)
      throw new UnsupportedOperationException("in define mode");
    spiw.updateAttribute(v2, att);
  }
Update the value of an existing attribute. Attribute is found by name, which must match exactly.
You cannot make an attribute longer, or change the number of values.
For strings: truncate if longer, zero fill if shorter.  Strings are padded to 4 byte boundaries, ok to use padding if it exists.
For numerics: must have same number of values.

@param v2  variable, or null for global attribute
@param att replace with this value
@throws IOException if I/O error