1. gpmc.msc (or administrative tools > Group Policy Management)
2. In the GPMC, expand out the domain and then go to Group Policy Objects
3. Right Click the Default Domain Policy and select Edit.
4. Go to Computer Configuration\Windows Settings\Security Settings\Account Policies\Password Policy and modify the Setting: Password must meet complexity requirements
5. On the server, from the command line, run gpudate /force
You may need to restart the server to have the policy take affect.