Are you talking about new updates for Windows Defender?
If so, you can update two ways: 1) through Windows Update - the Windows Defender updates will be presented, 2) open Windows Defender, click the down arrow next to the question mark, select "Check for Updates."
EDIT: If you are talking about updates for the anti-virus program Bitdefender, I'm certain that you can get them but I don't use the program and don't know the steps.