Within the header of your Testament theme there are several options that help you to setup your content easily and from one easy to follow section which will be accessible on every page within your themes editor.
In your Shopify admin open: Themes > customize theme
On the left side of the page click on the Header section to reveal the following options:
Logo - add your own custom logo image or alternatively if you do not have a logo as yet you can show just the name of your store plus configure it's text size, weight and case.
Promotional text - by adding text to this section you can alert your customers of any promotions, notices or simply welcome them to your store. Leaving this field blank will remove that message.
Cart icon - switch between a Cart or bag icon. Note that the terminology across your store will require changing should you wish to use Bag... Editing the language of your theme via the Online store > themes > menu will give you access to the language editor.
Navigation - choose a main menu for your store and configure it's layout options which include a mobile only style for those who require a minimal look to their store.
Mega Dropdown - Within your navigation you can also add a mega menu to links in your main menu. This mega menu can show multiple menus from one single drop down, for detailed instructions please see:
How to configure a mega dropdown menu