Currently we don't have the facility for merging two different accounts.
You can change the email address of the Qwiklabs account following the below steps:
1. Login to Qwiklabs with your credentials.
2. Click on the Gravatar icon at the top right corner of the main screen. Now, click on Settings
3. Change the email id from the profile page. (Make sure you are using the email Id which is not being used to create any Qwiklabs account till now).
4. Click the Update Settings button (If you are getting redirected to the same page after changing the email don't worry and follow the following steps). Then log out of the current session.
5. Go to the inbox of the new email id. Click on the confirmation link received.
6. You can now try to log in with your new email Id.