The correct method of changing your website menu depends on the design theme that you use. Pick one of the methods described in this article which is appropriate to your case.
Design theme settings
Look through the available settings provided by your design theme, they might offer a setup option which you can use to make the desired changes. Below is shown an example of such a setting option for "Default 3.0" theme:
If you know that the main menu of your website is created using the $wa->apps() method, then you can add a fixed set of links to your menu, with the desired URL for each link, in the "Settings" screen of the Site app. To do so, switch the $wa->apps menu setting to "Set up manually" and change menu items as you need.
If neither your design theme settings, nor Site app settings allow you to change your website menu, then the menu should be generated by one of the HTML templates of your design theme, without taking those settings into account. In this case you need make changes to the source code of that template file using your knowledge in HTML and Smarty technologies. If you need assistance when editing the source code, please consult your design theme developer or any other qualified specialist.