How and where do I edit query posts?

Hello. I read to split a post in pages to put this code in the html of the post:

<!–nextpage–>

It slipt the post but there was no page 1,2 anywhere.

I then found this:

The problem is query_posts() function overrides nearly everything in the standard posts object query, including paged offset is.

For proper pagination with query_posts(). Use the following code:

Existing Code:
<?php if(have_posts()) : ?>
<?php query_posts(“category_name=CategoryName”); ?>
<?php while(have_posts()) : the_post(); ?>

Replace With:
<?php if(have_posts()) : ?>
<?php $paged = (get_query_var(‘paged’)) ? get_query_var(‘paged’) : 1;
query_posts(“category_name=CategoryName&paged=$paged”); ?>
<?php while(have_posts()) : the_post(); ?>You are done!

Where to I swap this?

In my admin? my server files?

I need the exact steps on what to do, like:

Admin Panel>settings>query posts….. Like that.

Thank you very much 🙂