The templates in the library that are delivered out-of-the-box cannot be edited. This is to ensure the template always reflects the default style and layout so we can push out new versions/changes when required (e.g. for a new feature).
If you wish to customize a pre-delivered template in the library (or any other template created in your library), you can do this two ways.
Create a new template. In the create Template process, select Copy from Existing and select the template you wish to customise. This will copy the content from this template and apply it to your new template.
During the Create Campaign process the template selected is copied, you can edit the template associated to the campaign without interfering with the original
After your new template has been created you are free to edit all content attributes in the template.