The data from a feed is mapped to a template through a feed mapping. Simply put, it specifies which columns correspond to which elements in the template, so that Creatomate can insert data from your feed into a template.
In order to change the feed mapping, go to a feed and click one of its dynamic elements (outlined in orange) in the right-side preview, then click any column to choose how the data should be mapped.
As this is an advanced feature, you probably don't need it unless you're creating highly-dynamic videos where you want to go beyond simply replacing text or images.
As a default, when you map a column and element, it depends on the element type how data from the feed is used. For instance, when you connect an image to a column, it assumes that you are providing a URL to an image. If you want to replace a different element property instead, you can use a custom matching rule.
You can create a custom matching rule by choosing "Custom" while mapping a column to an element, then typing the property name you wish to replace. For instance, to replace the background color of a text element, type "background_color".
You can even edit the keyframe values of a property. For instance, you can use the matching rule "background_color.0.value" to map the first keyframe of the background_color property. For more information on this, you can consult the JSON format documentation and use the source editor.