Find all files ending with ".txt" in the current directory and its subdirectories, and for each of them, execute the command "sed -i 's/\([A-Za-z]\+\) \([A-Za-z]\+\)/\2 \1/g' {}", where {} is the file name.
find ~ -name "*.txt" -exec sed -i 's/\([A-Za-z]\+\) \([A-Za-z]\+\)/\2 \1/g' {} \;