What should I do concerning the bank deposit with prohibited interest that is about to expire, some of its interest having been used to purchase an apartment?

You must stop dealing with interest-based banks and withdraw deposits from them immediately, because usury is forbidden by Islamic law. It is not permissible to place money in interest-based banks except out of necessity. As for the interest previously earned based on a fatwa (definition) or ignorance of the ruling, there is no harm in benefiting from it, as mentioned by Ibn Taymiyyah and Ibn Uthaymeen. However, any interest-based earnings acquired after knowing their prohibition must be disposed of by giving them to the poor and needy or for the general welfare of Muslims.