both verbs "أستبدل" and "أغير" are close to each other , in this context "أغير" may indicates your desire to change the entire sort ,In other hand "أستبدل" may indicates your desires to alternate goods with same goods "have the same properties" both goods belongs to the same type .

depending on the context you can say: أريد أن أغيّر البضائع but this can indicates to I want to make some changes on this products by treating it then in the context of shopping: is better to say: أريد أن أستبدل البضائع in the case you didn't like what you have bought