Try this. Using for /r to iterate through all subdirectories and then Using %%~nxI to get the name and extension of the file.

set "folder=C:\stackoverflow\"

cd /d "%folder%"

setlocal enabledelayedexpansion

set "pattern=-"

set "replace=_"

for /r %%I in (*.txt) do (
    set "file=%%~nxI"
    ren "%%I" "!file:%pattern%=%replace%!"
)

endlocal