By default, the Menu Item that links to your Page in the Main Navigation uses that Page's Title as a Name. However, you may want to set a different value for the Menu Item Name without affecting the Title.
From the Pages tab, click on the page who's name you'd like to change in the menu.
Click on the Advanced Tab.
Scroll down to Menu and click the checkbox to the left to activate it.
Set the value you would like to use for the Menu Item Name.