APEX: Compare Old and New Values in a Trigger

// Inside your trigger handler
public static void onBeforeUpdate()
{
    try
    {
       for(Account acct : Trigger.new)
       {
           Account oldAccount = Trigger.oldMap.get(acct.Id);
    
          if(oldAccount.FirstName != acct.FirstName)
          {
               // Value Changed 
          }
       }
    }
   catch(Exception ex)
   {
       system.debug(ex.getMessage() + '  ' + ex.getStackTraceString()); 
   }
}