1) You can manually change the data field headings, by typing over the
exising headings.
You can't use a heading that's the same a a field name in the source
data, so you can't change 'Sum of Sales' to 'Sales'.
However, you can add a space character at the end of the name, to make
it different: 'Sales '
2) You can't change the default settings for the data fields. If a field
contains blank cells, or cells with text, it will default to COUNT.
Otherwise, it will SUM.
There's a feature that changes all the data fields to SUM, in my pivot
table add-in, that you can download here:
http://www.contextures.com/xlPivotAddIn.html