How to update a claim in ASP.NET Identity?

I created a Extension method to Add/Update/Read Claims based on a given ClaimsIdentity

namespace Foobar.Common.Extensions
{
    public static class Extensions
    {
        public static void AddUpdateClaim(this IPrincipal currentPrincipal, string key, string value)
        {
            var identity = currentPrincipal.Identity as ClaimsIdentity;
            if (identity == null)
                return;

            // check for existing claim and remove it
            var existingClaim = identity.FindFirst(key);
            if (existingClaim != null)
                identity.RemoveClaim(existingClaim);

            // add new claim
            identity.AddClaim(new Claim(key, value));
            var authenticationManager = HttpContext.Current.GetOwinContext().Authentication;
            authenticationManager.AuthenticationResponseGrant = new AuthenticationResponseGrant(new ClaimsPrincipal(identity), new AuthenticationProperties() { IsPersistent = true });
        }

        public static string GetClaimValue(this IPrincipal currentPrincipal, string key)
        {
            var identity = currentPrincipal.Identity as ClaimsIdentity;
            if (identity == null)
                return null;

            var claim = identity.Claims.FirstOrDefault(c => c.Type == key);

            // ?. prevents a exception if claim is null.
            return claim?.Value;
        }
    }
}

and then to use it

using Foobar.Common.Extensions;

namespace Foobar.Web.Main.Controllers
{
    public class HomeController : Controller
    {
        public ActionResult Index()
        {
            // add/updating claims
            User.AddUpdateClaim("key1", "value1");
            User.AddUpdateClaim("key2", "value2");
            User.AddUpdateClaim("key3", "value3");
        }

        public ActionResult Details()
        {
            // reading a claim
            var key2 = User.GetClaimValue("key2");          
        }
    }
}

You can create a new ClaimsIdentity and then do the claims update with such.

set {
    // get context of the authentication manager
    var authenticationManager = HttpContext.GetOwinContext().Authentication;

    // create a new identity from the old one
    var identity = new ClaimsIdentity(User.Identity);

    // update claim value
    identity.RemoveClaim(identity.FindFirst("AccountNo"));
    identity.AddClaim(new Claim("AccountNo", value));

    // tell the authentication manager to use this new identity
    authenticationManager.AuthenticationResponseGrant = 
        new AuthenticationResponseGrant(
            new ClaimsPrincipal(identity),
            new AuthenticationProperties { IsPersistent = true }
        );
}

Another (async) approach, using Identity's UserManager and SigninManager to reflect the change in the Identity cookie (and to optionally remove claims from db table AspNetUserClaims):

// Get User and a claims-based identity
ApplicationUser user = await UserManager.FindByIdAsync(User.Identity.GetUserId());
var Identity = new ClaimsIdentity(User.Identity);

// Remove existing claim and replace with a new value
await UserManager.RemoveClaimAsync(user.Id, Identity.FindFirst("AccountNo"));
await UserManager.AddClaimAsync(user.Id, new Claim("AccountNo", value));

// Re-Signin User to reflect the change in the Identity cookie
await SignInManager.SignInAsync(user, isPersistent: false, rememberBrowser: false);

// [optional] remove claims from claims table dbo.AspNetUserClaims, if not needed
var userClaims = UserManager.GetClaims(user.Id);
if (userClaims.Any())
{
  foreach (var item in userClaims)
  {
    UserManager.RemoveClaim(user.Id, item);
  }
}