This is what George meant:

1) Go to General > Client Information;

2) On the input screen, scroll down to the section for Miscellaneous Info;

3) On the line for Preparer, select the preparer from the dropdown box.

As you already know, just because someone is assigned to a return, it doesn't mean that person is the preparer.  Ultimate, the person to be listed as the preparer is the one who signs the return and meet the definition under §301.7701-15(b)(1).

Signing tax return preparer. A signing tax return preparer is the individual tax return preparer who has the primary responsibility for the overall substantive accuracy of the preparation of such return or claim for refund.