Below you can find an easy explanation, how to reset sa account password.
Download the PsExec tool from the Sysinternals suite.
Open with elevated CMD or Powershell privileges and run the following command
CMD:
C:\PsExec.exe -s -i "C:\Program Files (x86)\Microsoft SQL Server\140\Tool\Binn\ManagementStudio\Ssms.exe"
PowerShell:
.\PsExec.exe -s -i "C:\Program Files (x86)\Microsoft SQL Server\140\Tools\Binn\ManagementStudio\Ssms.exe"
SQL Server Management Studio will start with a system account. In the Username field we will observe the user “NT AUTHORITY \ SYSTEM”