De ou Des

The phrase is changer de qqc. You could say je change les vêtements, but this would mean that you change something pertaining to the clothes. Like you have them made wider or longer to suit you.

Hi Phúc,

This construction outlines the rule that after the preposition de the partitive articles and the plural of the indefinite article are omitted,

i.e           de + des = de
                de + du = de
                de + de la = de
                de +de l’ = de

e.g.        La salle est pleine de gens (de + des gens) – The room is full of (some) people

                Je rève de beurre au lieu de cette margarine (de + du beurre) – I’m dreaming of (some)  butter instead of this margarine !
               
                Le plancher est couvert de sciure  (de + de la scuire) – The floor is covered with (some) sawdust

                Quand je manque d’argent je fais des économies (de + de l’argent) – When I’m short of (some) money I make some savings

N.B In this type of construction the singular indefinite article is retained

e.g.        Je manque d’un stylo, j’ai besoin d’une gomme
                                BUT
                Je me suis emparé de crayons - I grabbed some pencils. (ie the plural indefinite article (des)   has disappeared.

As Chris said, in the case you cited the verb form is changer de so hopefully the above explains why why the answer is "de vêtements" instead of "des vêtements”.

Hope this helps,

Tom