To easily change the permissions, just navigate to the WordPress install folder, and execute this two commands:

#Set all directories recursively to 755:
find . -type d -exec chmod 0755 {} +

#Set all files recursively to 644:
find . -type f -exec chmod 0644 {} +

If you are not in the current directory you will need to add the desired one after find, something like:

find /top/path/here -type d -exec chown www-data:www-data {} \;