How do I change a password for a person on trello that has ceased employment

[Adam Mondy]

Employee has left , password unknown

[Nic Brough -Adaptavist-]

Welcome to the Atlassian Community!

The simple answer is probably that you can not.  Trello uses Atlassian accounts for access, and these could be owned by the individual, or an "organisation", but only the individual can change the password on an account, no-one can do it for them.

Atlassian accounts have a unique email address for them, and it is that email that a "forgot password" link will go to.

So, if they were using their work email for their Atlassian account, you do have one option - you could try to log in as them, use the "forgot password" link and then get your email admins to open the resultant email so that you can edit it.

However, this is not a good thing to do.  I would strongly recommend that you simply remove their access to your Trello workspaces.