Where can I change the down jacket?

Generally changed down jackets can be changed in tailor shops.

Ordinary tailor shops can change down jackets, but it is more troublesome to change down jackets, and professional tailor shops are needed to change them.

Wear down jacket when it is cold in winter. Don't modify it too small to keep warm. Change clothes carefully, especially some big-name clothes with special fabrics, which require high workmanship. You should find a professional clothes changing shop to avoid secondary damage to your clothes.