Change File Timestamps

Single file

touch -d "$(date -R -r "$filename") - 2 hours" "$filename"

Modify a large number of files

find DIRECTORY -print | while read filename; do
    # do whatever you want with the file
    touch -d "$(date -R -r "$filename") - 2 hours" "$filename"
done

find DIRECTORY -exec touch -d "2 hours ago" {} +