You don't. Marketing Cloud templates are locked for a reason - so that people not comfortable with HTML wouldn't be able to mess them up.
What you can do is copy the code and create a new, HTML-paste type of a Email and paste it there. This way you will be able to modify everything in the fresh code paste.
This is covered in the January 2018 release notes.
Show Full HTML Option
When editing a template-based email or the HTML for a template, there is a new option to see the full, compiled HTML.
You no longer need to send a test of a template-based email or edit an
individual content block to view the source HTML.
When selected, the full HTML of the email is displayed in a view-only
pane on the left. A lock icon indicates that you can’t edit the HTML.
When editing template HTML, a new option at the bottom of the HTML
editor lets you toggle to the “full” view of the HTML. You can’t edit
HTML in this view, and block HTML is highlighted.