It does not belong to you, and it must still be returned. You having it in the house will trigger bad memories of a major sin, which is theft, and therefore, it is best for you to get it out of the house. Take it to a local charity, if you are not able to return it.
As for repenting, when you are praying, or in any time you are reciting dua, repent for your sins in general, and include this as well.
Remember that Allah ta'ala is the All-forgiving, and therefore be thankful to the Almighty for guiding you to the right path.
With prayers for your success.