To change the administrator account's password, connect to your server remotely through Remote Desktop (RDP) and follow the steps below:
1- Click on Start > Server Manager.
2- Click on Tools > Computer Management.
3- On the window that appears, open : Local Users and Groups.
4- Then click on Users.
5- Right click on Administrator then click on Set Password...
6- On the alert that appears, click Proceed.
7- Enter your new password, confirm it then click on OK.
You will find in this article some tips for choosing strong passwords and thus protect your accounts: How to choose a strong password?