How To Update DB Using Code First.Net Core

Teaching myself .Net Core web stuff.

I have a model and I have successfully created my DB.

Now, I am adding a new Field to my model/class. I want to preserve existing data in this table so do not want to recreate the DB/Table. How can I update the DB/Table>

I have tried using 'Update-Database' in the Package Manage Console but got an error telling my table already exists. Which I know it does. But I am updating not creating.

This is my model:

public class Subscription
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public long SubscriptionId { get; set; }
    public string SubscriptionRef { get; set; }
    public string CompanyName { get; set; }
    public string EmailAddress { get; set; }
    public string Salt { get; set; }
    public string Key { get; set; }
    public bool Disabled { get; set; }
    public DateTime DOE { get; set; }
}

This is my DBInitalizer

public class DbInitializer
{
    public static void Initialize(WorkerContext context)
    {
        context.Database.EnsureCreated();
        context.SaveChanges();
    }
}

Which is called by my startup.cs

public void ConfigureServices(IServiceCollection services)
{
    // Add framework services.
    services.AddDbContext<WorkerContext>(options =>
        options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
    services.AddMvc();
}

This is my DB context:

public class WorkerContext : DbContext
{
    public WorkerContext(DbContextOptions<WorkerContext> options) : base(options)
    {
    }
    public DbSet<Models.Subscription> Subscriptions { get; set; }
}

So now I add a new field to my Subscription Table/Model:

public string Test { get; set; }

I rerun the project (i also type in Update-Database) and no new field is created...

How do I update/edit/modify/delete tables and fields using code 1st techniques?

Thanks