Development versions are pre release versions of ADE. They generally have new content and bug fixes. They are likely to be less stable than the production version. Eventually they replace the production version and then there may be a new development version. This page provides a download link for the current development version. This installation package may be used to update an existing development version or install it as new. The same package can be used for both purposes. You may have been directed this page because ADE has told you there is an update.
The current development installation package can be downloaded from here