To release a claim, head to
Dashboard > Monetization > YouTube Content ID, click
Claims and click on the Claim ID to open up the claim. Here you can see information about the claim, any information provided by the video owner, if they've disputed it, and you can also see the video itself.
On this page you can release a claim by clicking the Remove Claim button in the top right.
Once you have released the claim you cannot reinstate it so make sure you're 100% sure before you do so.
Content ID is available to Platinum members only. If you have a free account, you can upgrade here.