"You have changed a lot."
Translation:Tu as beaucoup changé.
"beaucoup" is not an adjective, but an adverb.
In this sentence, the tense is compound past ("vous avez changé").
Here is a link that explain the rules to determine the place of adverbs in French:
Short adverbs that modify a verb usually follow the conjugated verb. (Remember that in compound tenses, the auxiliary verb is the conjugated verb, so the adverb follows that.)
Nous mangeons beaucoup.
Nous avons beaucoup mangé.
Update: I just found out that I was wrong!! Because the verb only agrees with gender/plurality with the auxillary verb "etre" and not with "avoir" (as in this case).
Just updating for anyone who's confused
i.e. Il a changé --> Elle a changé --> Ils ont changé (He/she/they has/have changed)
but Il est parti --> Elle est partie --> Ils sont partis (He/she/they has/have left)
Read more here: https://www.duolingo.com/comment/38735