1. Replace parts: check whether the seat joint is loose or broken. If so, you need to replace related parts, such as screws, nails, boards, etc.

2. Reinforce the joint: If the joint is not obviously damaged, consider reinforcing the joint. Glue the joints together with wood glue and wood strips, and then fix them with screws or nails.

3. Replace the backrest: If the backrest is damaged beyond repair, it needs to be replaced with a new backrest. You can choose to buy the backrest of the same model as the original chair, or you can design and manufacture it yourself.