"They would need to change their clothes."
Translation:Ils auraient besoin de changer de vêtements.
To understand the structure of this sentence you need the following:
In French certain verbs are followed by the preposition de + noun. Changer is such a verb
structure: changer + de + noun without a determiner
Je vais changer de chemise - I am going to change my shirt
changer de chaussure - to change into other shoes
changer de maison - change houses
changer de train - change trains