Why my volume down button not working

@eshaankumar sounds like an issue with the volume flex cable. You are looking for something like this cable Example only not an endorsement of the seller.

Since iFixit does not yet have a specific guide for this use something like this video or the written guide from here It is for a 54s but will require identical steps. It is written in Spanish so use a browser that does the translating for you.

While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same issues with their device. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Repair is War on Entropy!