Find all files with the extension .txt and execute the command sed to replace the date format DD/MM/YYYY with YYYY-MM-DD for each file.
`find . -name '*.txt' -exec sed -i 's/\([0-9]\{2\}\)\/\([0-9]\{2\}\)\/\([0-9]\{4\}\)/\3-\2-\1/' {} \;`