1. Find the same material as underwear, cut it according to the original stitching, and then sew it, so that the back of underwear can be lengthened.
2. Replace the new back button. If your underwear is suspender style, you can try to replace the new back button and lengthen the length of the back button, which will increase the length of the underwear back.
3. Find a professional tailor's shop or underwear shop and ask if you can provide the service of lengthening the back of underwear, so that you can get professional help.
I hope the above methods can help you, but please be careful not to modify the structure of underwear at will so as not to affect the wearing effect.