First of all click Customise in the Admin menu bar. 

You will now see a new menu bar open on the left side of the screen with a long list of options. 

Scroll down the list of options until you see the Menus tab. Now click on the Menus tab.

Now click on Main Menu

This will open a full list of your current menubar options. 

Now scroll down & locate the page or item you would like to remove (we will remove a gallery page as an example)

Clicking on the item will open up a new box with the option to Remove from the menubar. 

Click Remove and then Publish

Your page has now been removed and will no longer be displayed in the menubar 👍

Also see
​Adding a new page to the menubar
​Changing the name of a menubar item