@@echo off
setlocal enabledelayedexpansion
set /p brewer="Enter Brewer #: "
set "folder=C:\SHICrivm\uvdata\%brewer%"
for %%a in ("%folder%\*_*.%brewer%") do (
    set "oldname=%%~nxa"
    set "newname=!oldname:_=!"
    ren "%%a" "!newname!"
)