Microsoft: Change Local Administrator Password of Windows or Windows Server

These two methods allow you to change the local administrator password of Windows or Windows Server:

<< Method 1 >>

1. Log on to the computer using the Administrator account.
2. Click Start, click Control Panel, and then double-click User Accounts.
3. In User Accounts, in Make changes to your user account, click Change your password.
4. In Change your password, in Current Password, type your password.
5. In New password, type a new password.
6. In Confirm new password, retype the password.
7. In Type a password hint, type a word or phrase that will remind you of your password or, optionally, leave this field blank.
8. Click Change password.

<< Method 2 >>

1. Log on to the computer using the Administrator account.
2. Click Start, right-click Administrative Tools, and then click Open. Administrative Tools opens.
3. Double-click Computer Management, click Local Users and Groups, and in the details pane, double-click Users. The Users folder opens.
4. In the details pane, right-click the account that you want to change, and click Set Password. A warning dialog box opens. Read the information to determine whether you want to proceed with the step to change the password.
5. In New Password, type a password. In Confirm password, retype the password, and then click OK.

Reference:
Change the Administrator Password
http://technet.microsoft.com/en-us/library/cc754651(v=ws.10).aspx