First, log in as Administrator user to your Windows Server 2012 R2. Then press Ctrl+Alt+End all 3 buttons at once. This will show you the option to change your password. You can see that in the image below.
Click the Change a password option. It will take you to the window to set a new password. Enter your old password and your new password as shown in the image and press enter for it to take effect.
You can logout and login using the new password now.
Some tips for setting a new password:
- The new password should be of at least 10 characters
- Use a combination of number, uppercase, lowercase, and special characters
- Do not use the previously used password
Do not include your first name, last name, or birth date in the password to make it more secure