The login email address for SuperSafe Boost is usually the email address you registered with which for most customers was the original MyAccount email login. So, if you've not changed the MyAccount login email then the login email address is still the same as you use for MyAccount.
Login to the SuperSafe Account here and if you don't know the password then click on Forgot your password? Of course all of that working depends on having access to the email address that's used as the SuperSafe Boost login.
What you need to do when logged in to your SuperSafe Account is release the licence for the pc. The account will still have that licence allocated. Then request a new licence for the pc and you'll be asked to download new SuperSafe software to the pc.
Of course if you've already downloaded before allocating the licence you may need to remove that software with the normal pc uninstall process. If for any reason there are uninstall issues then there's an f-Secure removal tool available.