A page's title can be edited by editing the page. When you click on the edit link, you will see a text box labeled "Title of the page:".
For example, if you try it on this page, you will see that this page is titled Page Title - How to edit?
Titles vs. Names
A page's name is not necessarily the same as its title. On creation of a page (either via the "Add a new page" box or from an open link in a page) the title will default to the page's name, but it can be modified at any time.
Though a page's title can be changed, you should take care not to confuse the reader by giving it a title that is completely different from the page's name.
Pages without a Title
It is possible to have a page without a title; you do this by leaving the 'Title of the page:' box blank in the edit view.
However, keep in mind that if you do this, your reader may have a hard time determining the name of the page, in particular if the address bar of the browser is hidden.
If parenting is used, the page's title will be used in the breadcrumbs. If a page does not have a title, its name will be used instead. (Parent settings are controlled by the +option button.)