You can run the following commands, a combination of find and chmod.
To change permissions to 755 for a directory and its subdirectories, run the following:
find DIRECOTRY_NAME -type d -exec chmod 0755 {} \;
To change permissions to 644 for files, run the following:
find DIRECOTRY_NAME -type f -exec chmod 0644 {} \;
DIRECOTRY_NAME: name of the directory containing subdirectories and files.