It's true what Stefano says, there's no out of the box action to convert sales order back to quote, however there's a couple of ways to achieve same thing.
1. You can simply use Copy document feature to copy the content of sales order back to quote and if Sales order is not shipped or invoiced simply delete the sales order. 2. Another way is simply to delete sales order and if document archiving is activated (Sales & Receivables Setup-> Archive Orders), deleted sales order will be stored in Archived Sales Orders, so once customer comes back you can again just copy it back from archive.
If this seems to be too much of the burden and you have frequent cancelations of sales order, I'd suggest logging an idea at ideas site to have such function included in future releases.
experience.dynamics.com/.../list
