We went over this a few months ago. In order to get Yellen's signature on the money a Treasurer had to be appointed. Biden appointed Lynn Malerba near the end of September. She then gave her signature to the engraver. Yellen's signature was already finished. Once the engraver finishes the plates new money can be printed with Yellen's signature as the Secretary of the Treasury and Malerba's signature as the Treasurer of the United States.

Money is printed with the former officer's signatures on it until new officers take office and their signatures are engraved and ready to be printed. That is why Mnuchin's signature is still on the cash - it takes a while for the engraving to be completed and Biden didn't pick his diversity hire (she is an Indian Chief) until June and she wasn't confirmed and sworn in until September of this year.