A list item can be replaced by assigning through its index.

List Update

ListUpdate.cs
using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        string replacement = ;
        List<string> names = new List<string> { "alpha", "beta" };
        names[1] = replacement;
        string joined = string.Join(",", names);

        Console.WriteLine($"replacement={replacement}");
        Console.WriteLine($"joined={joined}");
    }
}
using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        string replacement = ;
        List<string> names = new List<string> { "alpha", "beta" };
        names[1] = replacement;
        string joined = string.Join(",", names);

        Console.WriteLine($"replacement={replacement}");
        Console.WriteLine($"joined={joined}");
    }
}
using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        string replacement = ;
        List<string> names = new List<string> { "alpha", "beta" };
        names[1] = replacement;
        string joined = string.Join(",", names);

        Console.WriteLine($"replacement={replacement}");
        Console.WriteLine($"joined={joined}");
    }
}
indexed update An indexed update changes the item stored at one position.