How to add "previous" and "next" buttons

One good place to start is to take a look at the tutorials and user guide:

https://community.articulate.com/series/articulate-storyline-360

To add "prev" and "next" to your interface, Choose Player > Colors & Effects > Navigation: Icon and text

There are also many good resources right from the Articulate menu.  If you have a specific question about Next and Previous, feel free to ask.

The Prev and Next buttons automatically show up when Previewing your slide and are automatically selected when creating a new slide.

Thank you, guys!

click "Player", then "Aa Text Labels" -> Language

NEXT: line 123
PREV: line 145

Jürgen