You can make as many edits, changes, updates, tweaks or adjustments as you need to a preview render... these are FREE and UNLIMITED.
However, once you have purchased your HD render... it is final. There is currently no way to make change to a purchased render.
We only recommend purchasing a HD render when you have checked the video fully from start to finish (making sure everything looks and feels just right, as well as checking for typos if you have used text overlays)
If you have created an updated or improved version of the audio and wish to replace it on one of your past renders... this will be classed as a new video and will require a fresh purchase/credit to re-render.