Praise be to Allah.
‘Aqeeqah is a confirmed Sunnah, but there is no sin on the one who does not do it. That is because of the hadeeth narrated by Abu Dawood (2842) which says that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Whoever has a child born to him and wants to offer the sacrifice, let him do so, for a boy two similar sheep and for a girl one sheep.” The hadeeth was classed as hasan by al-Albaani in Saheeh Abi Dawood.
If you want to fulfil this Sunnah, then slaughter one sheep now, to complete what you did six years ago.
And Allaah knows best.