What software can I use to maintain and change content in older websites created with CS5?

Aside from old code possibly needing an update, there are a couple things that are specific to Dreamweaver that may be used in your sites that other programs will ignore. Specifically the use of Templates and Library Items.

DW uses specially formatted HTML Comment tags to make those features function within the program itself. Any other editor, aside from a version of DW, will treat those comments as simple HTML comments. 

Editing a Template.DWT file outside of DW, for example, won't update any of that Template's child pages in the rest of the site. 

Any modern code editor can be used to edit HTML, CSS and JavaScript code.  Although the code in legacy sites will likely need an overhaul to bring them up to current web standards and modern best practices.  Sites must be responsive and web accessible on ALL devices now.

Read chapters, do code exercises and take quizzes at the end.
- https://www.w3schools.com/html/
- https://www.w3schools.com/css/
- https://www.w3schools.com/js/

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CODE EDITORS:
-- Adobe Dreamweaver CC - https://www.adobe.com/products/dreamweaver.html
-- Atom (free) - https://atom.io/
-- Codespaces (free, browser-based) - https://github.com/features/codespaces
-- Nova (Mac only, formerly called Coda) - https://nova.app/
-- Pinegrow - https://pinegrow.com/
-- Sublime Text - http://www.sublimetext.com/
-- Visual Studio Code (free) - https://code.visualstudio.com/

Hope that helps.