How to replace a string in multiple files in linux from command line

find ./ -type f -exec sed -i 's/string1/string2/g' {} \;

For global case insensitive:

find ./ -type f -exec sed -i 's/string1/string2/gI' {} \;

Remove the dates from FileName extension

echo 'filename.txt_09232016' | grep -o '^.*\.txt'

Output: filename.txt