Did you read @johnebaker's reply? You need to look at the email you received when you purchased the upgrade. If you did not receive that email, check your spam folder. That email is your invoice and would include the download link for your software.
In the email with the serial number there is a link to the download.
Use that to download the installer program.
Run the installer, this will download and install the the main part program.
Start the program and activate it with the serial number and email address you used to purchase it.
You will then be offered to download the extra content which contains more features, functions and effects - do this - note this download is big and may take a while.
I would suggest, if you have the disc space, leaving the extra content in the install folder, you will be asked if you wish to do this, back them up after the installs are complete and remove if necessary - they will be in the folder Magix Downloads. This will save a lot of time should you ever need to re-install, .
Once the extras are downloaded the installs will start - you may need to acknowledge prompts for some of the extras installers.