1. Yöntem

In current release

Assuming you have handled the verification of the request to reset the forgotten password, use following code as a sample code steps.

ApplicationDbContext =new ApplicationDbContext()
String userId = "<YourLogicAssignsRequestedUserId>";
String newPassword = "<PasswordAsTypedByUser>";
ApplicationUser cUser = UserManager.FindById(userId);
String hashedNewPassword = UserManager.PasswordHasher.HashPassword(newPassword);
UserStore<ApplicationUser> store = new UserStore<ApplicationUser>();            
store.SetPasswordHashAsync(cUser, hashedNewPassword);

In AspNet Nightly Build

The framework is updated to work with Token for handling requests like ForgetPassword. Once in release, simple code guidance is expected.

Update:

This update is just to provide more clear steps.

ApplicationDbContext context = new ApplicationDbContext();
UserStore<ApplicationUser> store = new UserStore<ApplicationUser>(context);
UserManager<ApplicationUser> UserManager = new UserManager<ApplicationUser>(store);
String userId = User.Identity.GetUserId();//"<YourLogicAssignsRequestedUserId>";
String newPassword = "Test@123"; //"<PasswordAsTypedByUser>";
String hashedNewPassword = UserManager.PasswordHasher.HashPassword(newPassword);                    
ApplicationUser cUser = await store.FindByIdAsync(userId);
await store.SetPasswordHashAsync(cUser, hashedNewPassword);
await store.UpdateAsync(cUser);

2. Yöntem

UserManager<IdentityUser> userManager = 
    new UserManager<IdentityUser>(new UserStore<IdentityUser>());

userManager.RemovePassword(userId);

userManager.AddPassword(userId, newPassword);

3. Yöntem

 var user = UserManager.FindById(forgotPasswordEvent.UserId);

 UserStore<ApplicationUser> store = new UserStore<ApplicationUser>();
 store.SetPasswordHashAsync(user, uManager.PasswordHasher.HashPassword(model.ConfirmPassword));