Ui text to string ?

No need to Convert InputField to String - Because already InputField in string format.

String to Int :- int.Parse(varibleOfString);

Int to String :- varibleOfInt.ToString();

 using UnityEngine;
     using System.Collections;
     using UnityEngine.UI;
     public class TestCode : MonoBehaviour {
         public InputField x;
         string y;
         void Update()
         {
             y = x.text;
             Debug.Log (y);
         }
     
     }