An adjective is a word that describes a noun or pronoun. An adjective phrase is the combination of the adjective and the noun it is describing. In the example you were given, "scowling" is the adjective and "face" is the noun, so "a scowling face" is the adjective phrase.
If you needed to replace that adjective phrase with another that has a similar meaning, you would think of words that have meanings similar to "scowling" and "face." Your replacement phrases might be things like "a frowning mouth" or "a furrowed brow" or "a disgusted countenance."
If you have other sentences to rewrite, follow the same procedure. Identify the adjective phrase by finding the word that describes another word in the sentence. Add any articles that might be there to complete the adjective phrase. Substitute adjectives and nouns that have similar meanings, and you've got your replacement adjective phrase.