Say so, and say it early
If you are the one wearing it, it can feel impossible to say. But a ring you will wear every day for decades should be one you love, and the person who gave it almost always wants that too. Lead with what the ring means, then with what you would change: the way it sits, the metal, the size of the stone, the style.
If you are the one who gave it, try not to take it personally. The feeling behind the ring is what they said yes to.