Hi pianism,
Each time you change the value manually in database, you need to re-start your site.
I suggest you change the display name by code. admin is the user id, David is the new display name of this user.
CuteChat.ChatPortal portal = CuteChat.AppSystem.Instance.GetCurrentPortal();
CuteChat.IChatUserInfo info = portal.DataManager.GetUserInfo("User:admin");
info.DisplayName = "David";
Regards,
Ken