How do I remove a serialized field correctly?

 Hi guys,I save data with a serialized class ,now I want to remove a filed member that no more useful,but the old version has been already released,how can I prevent a exception when the new  version deserialized the UserModel?


 [Serializable]
 public class UserModel {
 // old version 
     Dictionary<string,int> readyToRemoveDict;
 }

 [Serializable]
 public class UserModel {
 //new version 
     //Dictionary<string,int> readyToRemoveDict;
 }
 
 //In gameManager ,that's how I saved it
 
     public void SaveData()
     {
         BinaryFormatter bf = new BinaryFormatter();
         FileStream file  = File.Create(Application.persistentDataPath + "/user.db");
 
         bf.Serialize(file,userModel);// this is the UserModel
         file.Close();
     }