A very generic answer as you have not told us which Magix programs(s) you have
The downloads and serial numbers are available from the My-Service-Centre, login with the email address and password you registered the software with.
On the My products page you will see your registered software with serial number(s), download link(s) and which computer(s) they have been activated on - deactivate these, download the program(s) again and re-install, activate and then download any extra content within the program where available or needed.