my nero has expired how can i download it again? it keep asking for serial number i don't have one to give .

Answer 1

Was this a trial version you had? If so, you will have to purchase it in order to keep using it. If it was one you had already purchased, it shouldn't have expired, and you should have the serial number, if you registered the program you could contact the vendor.

If it was a trial, even if you uninstall and reinstall, it still will read as being expired. Only possible loophole is to set back the calendar on your PC when you want to use it. This may or may not work.
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Answer 2

Have you tried reregistering it again?