One other thing worth noting is that the part count, performance cost, etc. are not updated, and will continue to show the original numbers.

1. Go to the “down arrow” button besides the download count, it contains the same menu where the “edit” button is located.
2. Click the “Update XML” button.
3. Try to choose a file. Note that files must have the same id as the craft you want to update.
4. Click upload, your SimplePlanes build post will be updated so users who click the download button will download the most updated version of the XML file you uploaded. You will also be taken to the “XML history” immediately, where you can view the list of XML files you uploaded to said build post.

This forum post sums up everything you asked in more depth and detail.
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I also have a number of concerns regarding this practice, most often:

• That you don’t get to update the thumbnails to reflect the current version of the XML file. Due to this, a build whose updated XML file is far too different from the thumbnail it depicts may be in violation of a rule about mislead thumbnails. Therefore, when posting new versions of XML files that is far different than the previous version, it is recommended you upload it as a new build post.