If one part of a video isn't right, you don't have to make the whole thing again. Open it in Studio and re-roll that layer on its own. The rest stays exactly as it was, and you only use usage on the part you redo.
Music re-rolls one track at a time. Pick a track in the tracklist and choose Re-roll this, or use Re-roll all above the list to redo the whole album. Visuals re-roll from the visual panel with Re-roll this. A re-roll uses usage for that layer only: one track for a track, and the picture for a visual.
Every visual re-roll is kept as a variant, so you can go back to an earlier scene, and pick the one you like under Channel & thumbnail when you publish. A re-rolled track is different: it replaces the old one, so re-roll music only when you're sure.
One re-roll runs at a time on a video. Press Re-roll while another is going and Studio asks you to wait for it to finish. If you don't have enough usage left, the re-roll doesn't start, and Studio tells you. If Extra usage is off, it offers to turn it on so your usage packs can cover it. If a re-roll fails or looks stuck, A render failed or looks stuck has what to do.