Menus are manually managed in WordPress. Log into wp-admin, and there are 2 places you can update these links.
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Appearance > Menus – select each menu in turn until you find the link in question, then update it. It’s best to add a Page (or Post or whatever content type) straight from this admin interface; then it pulls from the database ID and doesn’t hard-code the URL like you are running into here.
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Appearance > Customize – go to Menus, then same process, browse through all the menus until you find the problem links and replace them.