Browse to magix.com, click the Account login icon top right of the home page and login with the email address and password used to activate the software.
Under My products you will see your registered software with a download link and your serial number.
Note:- if you are re-installing to the same computer due to an issue you were having, then you need to clean up all the files that were left behind before re-installing in these folders
c:\Program files\Magix\ affected program
c:\Program files (x86)\Magix\ affected program
c:\ProgramData\Magix\ affected program - this is a hidden file so you may need to set Windows Explorer to show Hidden files.