The Catechism in Questions & Answers

543. Can the condition of the souls in the beyond be changed?

Yes. Since the sacrifice of Christ, it has been possible to change the condition of the souls in the beyond for the better.
After His death, Jesus Christ went into the realm of the dead and preached there. The preaching of the gospel implies an opportunity to change for those who accept it in faith.
Therefore a human being can also attain salvation after physical death.

Salvation: see Questions 243. et seq., 546.