I have found some code on how to create users in C# .

But I need to find a way to check if the user already exists by username or email - if he is already registered, I would like to be able to change his password and other data by code.

Below is the code used to create a new user:

private void AddUser()
{

DotNetNuke.Entities.Users.UserInfo objUser = new DotNetNuke.Entities.Users.UserInfo();
objUser.AffiliateID = Null.NullInteger;
objUser.Email = "user@email.com";
objUser.FirstName = "FirstName";
objUser.IsSuperUser = false;
objUser.LastName = "LastName";
objUser.PortalID = this.PortalId;
objUser.Username = "UserName";
objUser.DisplayName = "Display Name";

objUser.Membership.Password = "your_password";
//please check in web.config requirements for your password (length, letters, etc)
objUser.Membership.Approved = true;
objUser.Membership.Email = "user@email.com";
objUser.Membership.Username = objUser.Username;
objUser.Profile.Country = "Your Country";
objUser.Profile.Street = "Street";
objUser.Profile.City = "City";
objUser.Profile.Region = "Region";
objUser.Profile.PostalCode = "PostalCode";
objUser.Profile.Unit = "Unit";
objUser.Profile.Telephone = "Telephone";
objUser.Profile.FirstName = objUser.FirstName;
objUser.Profile.LastName = objUser.LastName;

DotNetNuke.Security.Membership.UserCreateStatus objCreateStatus =
DotNetNuke.Entities.Users.UserController.CreateUser(ref objUser);

if (objCreateStatus == DotNetNuke.Security.Membership.UserCreateStatus.Success)
{
  //everything fine
}
else
{
  //simple show error
  lblError.Text = objCreateStatus.ToString();
}

}

use the userinfo or usercontroller or membership (I dont remember exactly.... maybe UserController.GetUser) to fetch user ...... I think it will return -1 if user doesnt exist ..............if u fetch a user successfully by that username then user exists and you can update that user's info through the dnn api methods