How do I turn it off? I want to watch anime in a website on cellar data but I can’t because of this!

That isn’t your problem, it is the site’s problem. Either you misspelled the site name and a “lookalike” site exists with similar spelling (a practice known as “typosquatting”) or the legitimate site let their certificate expire. Either way, you should not go to that site until they fix the issue as it puts your identity at risk.