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It's a JavaScript file, you can use any text editor. You can also use the editor in the AlterVista control panel
If you use a CMS such as AlterPages or WordPress it's all ready to work on mobile...

Ok, that's called drop-down menu, a tab is different.
However, the culprit is the editor you used to create the website: it generates horrible broken code, especially for the drop-down menu.
To fix...

Tabs? Scrolling list? Could you please provide the URL of the page that shows the problem? With the banner on it, otherwise it's we can't verify.