How can I merge my Quickbooks trial online account with a new Quickbooks online account? I accidentally subscribe to a new account instead of upgrading the trial one.

Short answer is you cannot.  You have to abandon the new file and subscribe to the old one.  I would contact support to see what can be done with billing if you paid in full for the new one.  

There are online backup programs where you could backup your old file and restore to the new one but there are limitations with that too.  (aka Rewind app)

Lynda