You can rename it with the following exceptions:
1. The name can't be the same as the base field, so Count of Sep 06 can't be Sep 06 (but it could be Sep06 or Sep_06 or Sep-06, or even Sep 06 with an additional space after the word).
2. The name can't be the same as another name that you have arleady ued in that pivot table.
Those aside, you can rename a field by (1) selecting the field's drop-down list and (2) typing in a new name. You can also type in the formula bar to change the name, once the drop-down is selected.