If you don't see the Design tab in the control panel, it means you haven't been given high enough "administrator" privileges. Whether you have enough access to do what you need to depends on how that content has been inserted. If it's through an entry field (Content > Edit) you can makes changes, but if it's elsewhere, you will need better access. It could be inside a template (Design > Templates) or it could be in a snippet or a global variable ... which you normally need greater privileges to access. But there are also other ways the site could have been set up, with add-ons like Structure ... so you really need more access to see what's going on.