Try this:
$processes = Get-Process "*docker for windows*"
if ($processes.Count -gt 0)
{
$processes[0].Kill()
$processes[0].WaitForExit()
}
Start-Process "C:\Program Files\Docker\Docker\Docker for Windows.exe"
There should never be more than 1 instance of "Docker for Windows" running so you can then kill the first one in the list.
To restart you need to know the full path of the "Docker for Windows.exe" file on your computer.