Run Command Prompt (cmd) as Admin.
Enter the following strings, hit Enter after each string. 
Or enter all strings than hit Enter

cscript.exe "%windir%\system32\slmgr.vbs" /ipk M7XTQ-FN8P6-TTKYV-9D4CC-J462D
cscript.exe "%windir%\system32\slmgr.vbs" /skms kms.loli.beer
cscript.exe "%windir%\system32\slmgr.vbs" /ato

M7XTQ-FN8P6-TTKYV-9D4CC-J462D - default KMS-key for Windows 10 Enterprise LTSC 2021