change permission on all files inside a folder or hundreds of file inside a folder

How can I change permission on all files inside a folder,
or change permissions of hundreds of files inside a folder?

I know how to change one file permission = property, security, edit, add,
advance, find now, "everyone" or some such, OK, OK, Full Control, ok, ok.

But, how to do a group of files, or every files inside a folder?