First, find and open the pop-up you want to edit (from Components - EngageBox - List).  You'll see the same style of content editor you are used to. You can add headings, images, text, links, buttons, etc. Note that sometimes the styling will vary from the rest of the site and you need to keep the content short to fit within the defined window size.

Follow the instructions above to open the desired pop-up. Then, under the "Trigger" tab you can adjust how to limit impressions - the number and timing. You can adjust it so the visitor only sees it X number of times, or that once closed it stays closed for 1 hour, 1 day, 1 week, etc.

Under the Publishing Assignments tab you can adjust where to include or exclude the pop-up - based on the page, date and time, and many other variables. This allows a lot of flexibility, but be sure you're comfortable making these edits and check over your work to ensure the pop-ups behave properly.