Updating Mega menu

Navigate to Structure > Block Layout > Custom block library

From the Custom Block Library scroll to find the megamenu you wish to update

You can also use the filter available blocks by choosing Mega Menu from the block type dropdown and clicking on filter.

In the Mega Menu content is divided over 2 areas
Megamenu Menu (submenus)
Megamenu Links (Call-out sections)

Under Megamenu menu the sub-menus can be updated
 

Adding a page
New pages can can be added typing the name of the page you want to add into the URL field
The field will auto-suggest available pages matching your entry
Choose the page you want to add from the available options
If you want a label only (no link), add <nolink> to the URL field
Add your page label into the Link text field

Additional items can be added by clicking Add another item

Re-ordering pages
To change the order that the pages appear in the sub menu, drag the page upwards or downwards using the direction crosses to the left of each item

Under Megamenu link, 3 call out pieces of content can be highlighted
In the URL field the names of previously created pages will auto-suggest in this field as you type your choice.
Add link text to label the page you are linking to
Add a picture of dimensions 690x510px

When satisfied with changes, click save

To translate a menu for a different region that is accessible from the site, please review the guide on how to Translate Menu Items