Question

I used to wear a dress at home改为否定句

Answer 1

used to 是介乎于实义动词和情态动词之间的一个边沿动词，所以有 didn't use to (正式用法) 和 used not to (非正式用法) 两种否定式，所以这句话可改为以下两种否定句：

～I didn't used to wear a dress at home.
～I used not to wear a dress at home.

Answer 2

I used not to wear a dress at home.