There is only one account on this machine and it is Administrator. I want to change the content of a txt file in a folder that is marked "read-only". The folder was created when I downloaded and installed a purchased application. The process to change the attribute looks like it is working but the folder remains "read only".

That's because it probably also has its System attribute set. System files and folders are always read-only, regardless of any other attributes. You'll have to change that with the ATTRIB command in an elevated Command Prompt. But don't forget to change it back or the application may cease to work.