Bismihi ta'āla
They do a new nikah, and remarry.
I would advise that they address their previous issues, so as to not repeat them again.
And Allah knows best
Shaykh Dr Zaid Alsalami is an Iraqi born scholar, raised in Australia. He obtained a BA from Al-Mustafa University, Qom, and an MA from the Islamic College in London. He also obtained a PhD from ANU, Canberra. He has written and translated several Islamic texts and also prepared educational videos on Islamic rulings and practices.