i would like to change the tabs for example "home" to something else and would like to add a few new ones.
these things depend on the theme, and if you are lucky then someone will have the same theme installed, or is familiar with it, or is willing to download it and install it into a test blog (unlikely; see @esmi's remark).
'home' might be hard-coded into the menu code in header.php;
or your theme uses functions to build the menu;
or it has the new menu code and settings in admin;
the existing menu might be pages: then usually any new added page will appear automatically in the menu;
the existing menu might be categories: then usually any new added category will appear automatically in the menu;
the menu might be the new code with the option to build it in the admin area;
the menu might or might not be a dropdown menu, one or two levels, allowing to show child pages or categories;
with a link to your site, you are just more likely to get useful suggestions.