When we register as a non-joomla user and at back-end by admin through Components>dtregister>Records>user: linking this user with existing userID
Saved, but when it login through its username , still it shows new registration page, this user is not able to update his previous registration records where as he is already linked userid of joomla user group on the website.
I think you are confused as to how it works. When you go to the registration form, it doesn't matter if you've registered before or if you're logged in. You don't get anything else but the registration form.
To access the area to edit / cancel, create a Joomla menu item for DT Register and choose the User Panel option. This will give you a link to provide logged in users a way to view their registration history, make changes, etc.