What should I do with a large sum of money entrusted to me by a friend who has passed away, and he did not leave me any instructions regarding it, knowing that he deposited it with me 9 years ago?

If the amount was a trust (amanah) and your friend did not gift it to you, then it is not permissible to take it, and it must be returned to his heirs. If you are unable to identify them and reach them after thorough searching and becoming absolutely certain of despair, then give this amount in charity on behalf of the deceased.