Search the entire directory tree up to a maximum depth of 3 for any files ending with ".csv" and execute the command "sed -i 's;[A-Z];\L&;g' {} \;" on each of them. This command will replace any uppercase letters with their lowercase equivalents.
find / -maxdepth 3 -name '*.csv' -exec sed -i 's;[A-Z];\L&;g' {} \;