To merge all your badges from different account to one account in Salesforce Trailhead, check the below link

https://force.desk.com/customer/portal/articles/2418878-merging-trailhead-accounts?b_id=13478

Note:
Make sure you have the usernames of the accounts which are to be merged.

To link Trailhead & Webassessor accounts, check the below link

https://trailhead.salesforce.com/connect-webassessor?lang=en