How would i change this string in update to a StringBuilder?

I'm not sure why you want to do that. A StringBuilder only has benefits when used with really large strings. If you want to avoid garbage generation, that's not possible since the final ToString call that you need when using the StringBuilder will also create a new string and therefore garbage.

Anyways, this is what it looks like when using a cached string builder:

 StringBuilder sb = new StringBuilder();
 
 void Update()
 {
     // [ ... ]
     sb.Length = 0;
     sb.Append(hours).Append(" :");
     if (minutes < 10)
         sb.Append("0");
     sb.Append(minutes).Append(" ");
     minutesOClock.text sb.ToString();
 }

This should output the same as your string.Format version. Note that you could use AppendFormat for the minutes, however since the parameters of AppendFormat (just like string.Format) are object references, value types like int / ulong / etc would be boxed and create extra garbage.

Use CacheIntString class. It's made just for cases like this! Think of it as specialized hash table for strings.

It's core idea is to make you think how to create all your strings once and never allocate mid-frame again (or rarely at least). First thing you need to do is to split your timer into separate (repeating) numerical pieces. Then define how every particular piece will display in CacheIntString constructor calls like below:

 using System;
 using UnityEngine;
 using UnityEngine.UI;
 public class TestCacheIntString : MonoBehaviour
 {
     public Text _displaySeconds;
     public Text _displayMinutes;
     public Text _displayHours;
     public Text _displayDays;
     public double _time = 0;
     CacheIntString cacheSeconds = new CacheIntString(
         (seconds)=>seconds%60 , //describe how seconds (key) are translated to useful value (hash)
         (second)=>second.ToString("00") //you describe how string is built based on value (hash)
         , 0 , 59 , 1 //initialization range and step, so cache will be warmed up and ready
     );
     CacheIntString cacheMinutes = new CacheIntString(
         (seconds)=>seconds/60%60 , // this translates input seconds to minutes
         (minute)=>minute.ToString("00") // this translates minute to string
         , 0 , 60 , 60 //minutes needs a step of 60 seconds
     );
     CacheIntString cacheHours = new CacheIntString(
         (seconds)=>seconds/(60*60)%24 , // this translates input seconds to hours
         (hour)=>hour.ToString("00") , // this translates hour to string
         0 , 24 , 60*60 //hours needs a step of 60*60 seconds
     );
     CacheIntString cacheDays = new CacheIntString(
         (seconds)=>seconds/(60*60*24) , // this translates input seconds to days
         (day)=>day.ToString() , // this translates day to string
         0 , 31 , 60*60*24 //days needs a step of 60*60*24 seconds
     );
     void Update ()
     {
         _time += Time.deltaTime;
         int seconds = Mathf.FloorToInt( (float)_time );
         _displaySeconds.text = cacheSeconds[ seconds ];
         _displayMinutes.text = cacheMinutes[ seconds ];
         _displayHours.text = cacheHours[ seconds ];
         _displayDays.text = cacheDays[ seconds ];
     }
 }