Open Windows Defender (Start, Control Panel, Windows Defender)
Click the arrow next to the Help button, and then click Check for updates. If you are prompted for an administrator password or confirmation, type the password or provide confirmation.
If you don't use scheduled scans, or if you don't get updates automatically, you should check for new definitions at least once a week. To help protect your computer, Windows Defender will notify you if your definitions are out of date for more than seven days.