REM Renames all files with extension “TXT” to “DOC”
REN *.txt *.doc
REM Renames all files named “abd.extension” to “d.extension”
REN abc.* d.*
REM Renames all files named “a.extension” to “A.extension”
REN a*.* A*.*
REM Renames all files to “xxx6.extension”
REN *.* ???6*.*