Which program should I used to edit WordPress files?

If you are using the Plugin or Theme Editor, then use the View/Edit ability of an FTP program such as FileZilla.

Some hosts offer an editor ability also.

See also new free very good PHP IDE – https://www.codelobster.com
with WordPress plug-in available

Why would you do that when there’s lots of FREE programs that do the task perfectly fine.

Nice try pimping your software, just a shame it’s not FREE like MANY other apps…

As samboll discovered, $24.95 for the WordPress plugin:
https://www.ads-software.com/support/topic/246382?replies=3