If you don't need it, you can deactivate plugin Newsletter in Admin -> Plugins.
To change title & description for this section or remove it, you can change it in Admin -> Appearance -> Widgets
To translate other texts of this section, go to Admin -> Translations -> Theme translations.
You can modify it in Admin -> Appearance -> Widgets.
You can click on "Delete" button to remove widget from footer sidebar or change the name, selected menu.
If you want to add more widgets, just drag and drop from the left side to sidebar.