If a finished scene did not land the way you wanted, regenerate it. Open the tour, find the scene card, and press Regenerate. Avenue 510 renders a fresh take of that scene and it replaces the current one. The old version is not kept.
Regenerating renders a new clip, so it costs the same as a scene in a fresh tour. The cost is on the confirmation before you commit, along with the credits you have left.
The scene keeps its camera movement. Movement is chosen on the Motion step, before you generate, and it locks once the tour renders. If you want a different movement for that frame, remove the scene and add it back with the movement you want, or start a new version of the tour. You can also switch Ambient motion on the card first, and the regenerate is what makes that change take effect.
Opening a finished tour from your Media Library is a separate thing. The control there is Manage, and pressing it starts nothing and costs nothing: it opens that tour on its final step, where regenerating a scene is a further action with its own confirmation and its own cost.
If a single clip is worth building on rather than redoing, you can send it into a tour of its own instead. See building a tour from clips you already have.