How should I repair loose wire of the speaker of my headphones?

The wire inside has broken. It’s pretty complicated to repair and very possible to damage the headphone again while repairing. Because this headphone was not designed to be able to replace the wire and you have to use a soldering iron which needs you to spend some time to learn (if you have never use it before).

If you want to try anyway:

You need to remove the cover of the headphone with a screwdriver and use the soldering iron to remove the broken wire from where it’s connected to the headphone. Then, buy a new wire and use the soldering iron to connect it to the headphone.