How can a married couple expiate their sins resulting from their interactions as a couple during the engagement period?

The mercy of Allah is vast, and the servant should know that if he makes his deeds sincere for Allah and his repentance is good, this will be a reason for Allah to change his evil deeds into good deeds, in accordance with the Almighty's saying: ﴿Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful."﴾ and His saying: ﴿Except for those who repent, believe and do righteous work. For them Allah will change their evil deeds into good ones. And ever is Allah Forgiving and Merciful.﴾