Reboot your server, and with the F11 key, go to the boot menu:

Step 3 – Installation Window

When the installation window opens, click Next to continue.

Step 4 – “Repair your computer”

Select the “repair your computer” option.

Step 5 – “Troubleshoot”

Select the “troubleshoot” option.

Step 6 – “Command Prompt”

Select “command prompt”.

Step 7 – CMD

Inside CMD please type:

d:
cd Windows
cd System32
move utilman.exe utilman.exe.bak
copy cmd.exe utilman.exe
net user administrator /active:yes
shutdown -r -t 0

Step 8 – New Password

After server reboot, on the home login screen, please press the Win+U combination. Inside CMD window, please type:

net user Administrator your-new-password
exit

Step 9 – Rollback

Your password has been reset. However, you need to rollback your changes with utilman.exe.
Just reboot your server, and repeat this guide. Inside CMD window in Windows Recovery mode, please type:

d:
cd Windows
cd System32
del utilman.exe ren utilman.exe.bak utilman.exe
shutdown -r -t 0