Download template preview

I have a predefined template with some columns of data, basically should be grid. For example, There are columns like X,Y,Z and and user downloads the template. how do i show the preview of template(Columns headers available) before downloading so that if user is OK with columns he can download or should be able to edit/add new columns before downloading.

Concern here is User should be adding only data under the columns after the download of template but not column headers.

Any suggestions for this?