This depends on the material of the soffit. If the soffits are made of wood, they may be rotted and must be replaced. However, if they are made of vinyl or aluminum, they may have simply come loose from the frame. You may be able to remove the section that is sagging and then reinstall it better so that the nails or screws pull it tight again, although you should hire someone to perform this task if you’re not experienced at working with vinyl or aluminum panels.