The impeachment charges against Han have been dropped which has caused him to be reinstated as the interim President:

The Constitutional Court dismissed the impeachment of Prime Minister Han Duck-soo on Monday, reinstating him as acting president in the ongoing turmoil over suspended President Yoon Suk Yeol’s brief imposition of martial law.
Han’s impeachment was dismissed in a 5-1 vote by the court’s eight justices. Two justices voted to reject the impeachment motion entirely.
The ruling came three months after the National Assembly impeached the prime minister and then acting president over his alleged role in Yoon’s Dec. 3 declaration of martial law, among other reasons.
Four of the five justices who voted to dismiss Han’s impeachment acknowledged there were violations of the Constitution and the law in his decision to defer the appointment of additional justices to the court, but noted it did not justify his removal from office.
As the only justice to uphold the impeachment, however, Justice Chung Kye-sun said the violations were “serious” enough to warrant his dismissal.
You can read more at the link, but I think people should not read too much into this dismissal as it relates to impeached President Yoon Suk-yeol. None of the justices believed that Han had anything to do with the martial law declaration:
The six justices who voted to either dismiss or uphold the impeachment said there was no evidence to back the National Assembly’s accusations that Han took proactive action to give legitimacy to Yoon’s martial law bid, such as by convening a Cabinet meeting ahead of its declaration.
For Yoon to get his charges dismissed he needed to convince the judges that his martial law declaration did not violate the ROK constitution and laws which is very different charges than what Han was facing. With Han impeachment decision now completed it will be interesting to see if the Constitutional Court releases their decision on Yoon at the end of the week.