How to Change Categories without Plugin?

I have a theme with it’s own Categories widget.

For example, I can use the:

* Default WP Categories Sidebar Widget (or)
* My theme’s Categories Sidbear Widget

I’m using my theme’s widget for Categories because it streamlines better with my theme.

I have tried Categories plugins. But, those do not work on the Theme’s Categories. It only works on WP Categories.

SO, question:

How do I change the Unique Category ID without plugins?

“Way back when,” the ID’s used to be visible (in older versions of WP). But, now I can’t find them.

Thanks