What should I do after naming my daughter Retaj — she is now three and a half years old — and am I sinful, and what is the expiation?

Changing a name is not tied to a specific age; the Prophet, peace and blessings be upon him, changed the names of some companions even when they were old. Names with an ugly meaning are disliked (makruh tanzih) but not forbidden (haram). There is no sin upon someone who keeps such a name and does not change it. Nevertheless, changing them is preferable.