input: Write a python function to change the labels of form fields in a Django form.
  assert change_field_labels(SignUpForm(), 'username', 'New Username') == 'New Username'
  assert change_field_labels(SignUpForm(), 'email', 'Contact Email') == 'Contact Email'
  assert change_field_labels(SignUpForm(), 'password1', 'New Password') == 'New Password'
output: def change_field_labels(form, field_name, new_label):
      form.fields[field_name].label = new_label
      return form.fields[field_name].label