In the Newsletter 1 and Newsletter 2 templates, you can use the preset content regions, change the content region, add content to blank regions, or drag and drop new content areas.
In Salesforce Marketing Cloud Content Builder Classic Templates, how do I add or change newsletter content regions?
Hover over the content region you want to change and click on the arrow in the upper right-hand corner. Select Delete.
Confirm that you want to delete the content block. The content region will be empty.
In the right-hand pane, click on the Content tab. Click on Folders, go to Shared Folders > IU Assets > Newsletter 1 Content or Newsletter 2 Content.
Find the content mockup or placeholder you want and drag and drop it into your empty content region.