Coin Collecting: Updating UI Text

First you need to cast it as a string, then I would change the way you're handling these, seeing how you're mixing up things that should be static and not using proper patterns even though it does look like you're trying to. You want your coins to handle their own logic, you don't want your Coin Collect to handle things like rotation, so:

 using UnityEngine; 
 using System.Collections; 
 using UnityEngine.UI;
 
 public class Coin : MonoBehaviour
 {
   public int coinAmount;
 
   void Update() {
     transform.Rotate(new Vector3(0, Time.deltaTime * 50, 0));
   }
 
   void OnTriggerEnter(Collider other) {
     //Don't use tags, they're slower
     if (other.gameObject.name == "Player") {
       AudioSource source = GetComponent<AudioSource>();
       source.Play();
       UpdateScore();
     }
   }
 
   static void UpdateScore() {
   //You want this to be static because it's a function shared among all the Coins so only one instance of it. 
     CoinCollect.totalCoins.text = (string)(CoinCollect.currentCurrency += this.coinAmount);
   }
 };
 
 public class CoinCollect : MonoBehaviour {
 
   public int minCoin = 100, maxCoin = 500;
   public static GUIText totalCoins;
   public static int currentCurrency = 0;
   
   void Start() {
     Coin coin = new GameObject("coin").AddComponent<Coin>();
     //Put your Coin gameobject where ever you need it
     coin.coinAmount = Random.Range(minCoin, maxCoin);
     this.totalCoins.text = (string)currentCurrency;
   }