@echo off
setlocal
for /f "usebackq tokens=*" %%a in (`dir /b /a:d`) do (
  rem enter the directory
  pushd %%a
  echo In Directory: %%a
  for %%a in (*.xlsx) do ren "%%~a" "2018_JAN_%%~a"
  rem leave the directory
  popd
  )
endlocal