I have some basis question about rasa, could you experts give me practical advice

My questions is as follows:

  1. How to do validation of user input which name is slot?
  2. How to go back the dialogue initial status when the following cases has been occured:
  1. One round dialogue completed;
  2. it need user to re-enter his slot value when user input is invalid 3)I want to re-initiate the conversation anytime and anywher while conversion is on going…

I am new for rasa, your advice will be appreciated.