Is there any way to post a review using a registered alias rather than one's username?

No.  Usernames are used for all reviews, and aliases may be used for posting if you wish to remain anonymous and separated from your reviews when participating on the discussion boards.

then can I change my username...please?

You can PM or email TER Support ([email protected]) with a request to have your TER account username changed.

Current username format requirements:
- maximum 16 characters
- alphanumeric characters only with no spaces
 
If you change your username, all existing posts AND reviews under the previous username will reflect the new name.  Existing posts made under an alias will not be affected.