You need to complete her laundry day event. There will be an info text saying that you can have her change clothes.
Have a look at my gameplay guide (Linda Stage 4 - Laundry day): https://docs.google.com/document/d/1RiddNpybmX089L_Ib_91Vdi4R8bLZWPqTnTvz3N5QnY/...