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 amend (we will amend the name of the gallery page as an example)

Clicking on the item will open up a new box with the option to amend the Navigation Label. 

Type the new name that you'd like displayed (you'll notice the previous original name will be shown underneath)

Once happy, just click Publish

The label for the page has now been updated in the menubar 👍

Also see
​Adding a new page to the menubar
​Removing a page from the menubar
​