In order to download an eBook, you must install Adobe Digital Editions: www.adobe.com/nl/solutions/ebook/digital-editions/download.html. To access your eBooks with Adobe Digital Editions, take the following steps:
Install the Adobe Digital Editions app.
Log in with your Adobe ID. If you don't have this yet, you can create it in the app.
Click on the download link you received from AUP by email. You will then receive a license in the form of an .acsm file.
Adobe Digital Editions will verify your license and then download the eBook in ePub or PDF format.
You can now start reading.