You can't modify "original" templates (predefined, downloaded or purchased in marketplace)
The mains reason that "original" templates can't be modified is further template autoupdates and addons upload will be automatically. So your changes would be overwritten by new update source code.
To avoid that,
you have to CLONE your template
and start customizing it through Clone option in Template editor.
There is also, Create option, that allows you to create variations of a template and save as a new one, having the original as a "backup".