I am studying elementary Japanese, not classical forms of the language. In the online dictionary Jisho.org, I found the verb 憂ふ/'to be worried or concerned about' listed as a nidan verb. Does that mean that the verb must be used/conjugated in classical Japanese, that it would now be integrated as an ichidan verb, or that it is completely archaic and would not be used at all now?
That being said, since this is a literary verb, the classical version is somethimes used today using the modern kana orthography (憂う). For example, 憂える声 and 憂う声 are both used ("voice of concern"). The former is theoretically correct but the latter is preferred by many. Practically speaking, you should be using the modern version when you want to conjugate freely or stick to the standard (憂えている, 憂えた, ...), but as an exception, you may use unconjugated 憂う at the end of a sentence or a relative clause.