Header/Footer Sections
Header Settings
From Sections tab of Customize (Theme Editor) > select Header section

Section will listing all options to control:
- 
Logo Shop
 - 
Menu setting
 - 
Contact setting
 - 
Social Network setting
 - 
Mega menu setting
 

Dropdown menu

Please follow the guide at here
Mega menu

You can added unlimited Mega menu with multi-columns from 1 to 6 columns.
Each Mega Menu have column types:
- 
Navigation (Link list) with 1 or more Links
 - 
Product
 - 
Article
 - 
Custom block (HTML content)
 - 
Image
 
To setup Mega Menu, you need go to Header section, scroll to to the bottom, you will see the Content area:
- Step 1: click to 
 and select Sub Menu Setting 

The Important is Target field and Number of column field. You must input the menu name that existed in the Main menu into it.
Example: Your currently Main menu is: HOME, NEW IN, WOMEN, MEN, BABY & KIDS, SHOP, PAGES, BLOG . And want setup menu Women is Mega menu. You need input the text Women into the Target field 
Then select the column(s) that you want show in Mega menu conent. Select in Number of column selectbox.
You also can config the Background of Mega menu.
- Step 2: click to 
 and select Sub Menu Content 

The Important is Target field. You must input the menu name that input in Step 1. Continue example is Women text
Then you can config these options: Navigation 1 link, Navigation multi links, Product, Article, Custom block and image.
Example create the Women menu have Mega menu with 3 columns and content is Navigation multi links (2 sub-links) and image.

Result of example:

Do the same way for other menu want become Mega menu.
Troubleshooting
Login/Register or User icon Link don't appear
Your Shop setting currently is disabled Customer account.
From your Shopify admin, go to Settings > Checkout > Customer accounts and select option to enable Customer account such as Accounts are optional or Accounts are required
How to setup Currency
Please follow the guide at here
How to setup Navigation (Menu)
Please follow the guide at here
Mobile Menu
From Sections tab of Customize (Theme Editor) > select Mobile Menu section

You will see options to control the Mobile Menu at bottom of the screen
- 
Mobile Logo
 - 
Main menu (... More icon)
 - 
Contact Information
 - 
Social icons: follow setting at here
 

If you want change the icons, please submit new conversation.
Footer Settings
From Sections tab of Customize (Theme Editor) > select Footer section
Section include these blocks:
- 
Newsletter: These fields for this block content: Display on, Grid For Desktop (Mobile will show full width), Title (Heading), Mailchimp form action URL ( how to get it here), Placeholder text and Submit button text (Action text).
 - 
Logo and Copy right: show Logo, Copy-right text and Payments listing.
 - 
Link list: show navigation.
 - 
Contact & Social
 

Click to each block content to control the text/settings.