Note that if you disable/uninstall a module, you will lose the functionality provided by that module. But it looks like for special_menu_items, some of their functionality is in core now (assuming this is the module in question: https://www.drupal.org/project/special_menu_items).
I'm not as familiar with drupal 8, but in 7 to disable the module you can simply go to the modules page and do it there. You can also do it on the command line with drush if you have that installed: "drush dis special_menu_items".
Note that disabling doesn't remove the module, it just turns off the functionality. To actually remove it from your site entirely, you need to remove the code files themselves. Some modules also have an uninstall option, which allows you to remove its data from the database first; in d7 this is again found on the modules page. Whether you run an uninstall on a module or not, to then remove the module files you need to connect to your hosting server (ftp/cpanel/ssh), find the modules folder, and remove the module. In d7 you would (usually) go to sites/all/modules then remove the special_menu_items folder.
Now having written all that, a quick google search indicates that d8 has maybe changed this process. However most of the search results that came up for me were 1-2 years out of date at this point.
Hope this helps...
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Extend > Uninstall module