Below is the official document for Atlassian Bamboo serving this topic:
https://confluence.atlassian.com/bamboo/restoring-passwords-to-recover-admin-users-289277354.html

It provides description for internal DB HSQLDB only. As Bamboo versions higher 6.3 use H2 as internal DB, this part is not covered by official documentation (see above).

The below Atlassian article shows how to deal with an internal H2 DB, but based on Jira.
https://confluence.atlassian.com/jirakb/accessing-jira-s-h2-embedded-database-776818136.html?_ga=2.238456768.172662191.1523279672-1311615398.1523044491

I t"translated" the Jira one for Bamboo usage and linked it with the one of the first link so to support a customer inquiry with the need of admin password reset.

Please add an appropriate H2 part to the existing official Bamboo documentation.
Thanks a lot.

Workaround

https://confluence.atlassian.com/display/BAMKB/Accessing+Bamboo%27s+H2+embedded+database