Select your Ad-Owl Url. For most sites this will be "place_an_ad" (some sites use a URL that is named differently or under a top level url).

Double Click this url to load it in the Design / Blocks

Expand the Page Customizations Panel, on the right side of the page.

Inside the Ad-Owl tab check "Welcome text" this will make the text editable. Make any desired modifications.

Be advised there is a text limit of 650 characters. You can use html to add inline styling if you choose.