You can just go to a large regular local shopping mall, where there will be a special jewelry department and a jewelry processing place. Generally it can be changed. Go to the mall and ask the salesman who sells jewelry directly where there is a processing place.

Supplementary answer: It is safer in big shopping malls. You can ask the processor to take down the diamond and hold it yourself. You should be watching. Give it to him when the ring bracket is finished. He can press it. You can see the process of taking diamonds and installing them next to them.