If you want to disable password complexity policy in Active Directory Windows Server 2008, you have to do it using Group Policy:
1. Open Group Policy Management Console (Start > Run > gpmc.msc).
2. Open the Domain, and right-click and Edit the "Default Domain Policy".
3. Go into the "Computer Configuration" > "Windows Settings" > "Security Settings" > "Account Policies", and modify the "Password must meet complexity requirements" setting.
4. Start > Run > gpupdate /force to force update the group policy.
Now you can change the user's password to whatever you want.