UpdateGitHubBranch.cs

using Octokit;
using System;
using System.Threading.Tasks;
using System.Diagnostics;


namespace UpdateGitHub
{
    class Program
    {
        static async Task Main(string[] args)
        {
            string token = string.Empty;
            string login = string.Empty;
            string repo = string.Empty;
            if (args.Length == 2 && !string.IsNullOrEmpty(args[0]) && !string.IsNullOrEmpty(args[1]) && !string.IsNullOrEmpty(args[1]))
            {
                token = args[0];
                login = args[1];
                repo = args[2];
                
            }
            else
            {
                token = "YOUR TOKEN";
                login = "YOUR LOGIN";
                //Optional if you only want one repo
                repo = "Repo You want to Change";
            }

            var client = new GitHubClient(new ProductHeaderValue("update-github"));
            var basicAuth = new Octokit.Credentials(token);
            client.Credentials = basicAuth;

            var user = await client.User.Get(login);

            var repos = await client.Repository.GetAllForUser(login);

            foreach (var repo in repos)
            {
                if (string.IsNullOrEmpty(repo.Name) && repo.Name == repo)
                {
                    if (repo.DefaultBranch != "main")
                    {
                        LibGit2Sharp.Repository.Clone(repo.CloneUrl, @"C:\dev\Temp\" + repo.Name);

                        RunCmd(repo.Name, "git branch -m master main && git push -u origin main");

                        RepositoryUpdate update = new RepositoryUpdate(repo.Name);

                        update.DefaultBranch = "main";
                        await client.Repository.Edit(login, repo.Name, update);

                        RunCmd(repo.Name, "git push -d origin master");
                    }
                }
            }
        }

        private static void RunCmd(string repo, string cmd)
        {
            var processStartInfo = new ProcessStartInfo
            {
                FileName = "cmd.exe",
                Arguments = "/C " + cmd,
                RedirectStandardOutput = true,
                UseShellExecute = false,
                WorkingDirectory = @"C:\dev\Temp\" + repo
            };
            var process = Process.Start(processStartInfo);
            var output = process.StandardOutput.ReadToEnd();
            process.WaitForExit();
        }
    }
}