Method 1
- For Windows  XP and 2000 users, open and copy from the following location:
    
C:\Documents and Settings\All Users\Application Data\Malwarebytes\Malwarebytes' Anti-Malware\rules.ref - Windows Vista and Windows 7  users, open and copy from the following location:
    
C:\ProgramData\Malwarebytes\Malwarebytes' Anti-Malware\rules.ref 
Method 2
Download a manual definition updater here