You need to edit your theme’s page templates where ever you want the next/previous links to appear as page links. Thus it’s hard to be specific without knowing your theme or on which templates you want the page links to appear.
Before you start editing theme templates, you should create a child theme so your changes are not lost when your theme updates. Initially, you just need the style.css page with the proper header and an @import line for the parent stylesheet. When activated, your site under the child theme should not look any different than under the parent theme.
Once your child theme is working, you can copy over any templates you want to change. If you tell us what theme you are using and where you want the page links to appear, perhaps we can be more specific.
