There is no such thing as a subscription to iTunes. iTunes is simply a piece of software on your Mac. It is included at no charge.
Perhaps you subscribe to iTunes Match. If so, you unsubscribe by using iTunes' menu to go to Store > View Account. Sign in, go to the iTunes Match row, and click "Turn off automatic renewal."
Or perhaps you subscribe to Apple Music. If so, go to Store > Apple Music to unsubscribe.