On an editing page, scroll down and look for the Related page ads box (you may need to click the arrow to reveal its content) 
Click select
Search or browse for your preferred page ad and click it
Dont forget to Publish/Update your post!
Choosing a specific page ad to display in the sidebar
Page ads are displayed in the sidebar of each page below the In this section menu. These page ads are chosen at random, but you can choose a specific page ad to display at the top.