Currently I don't think it's possibile. If you lose the email that Forge sends upon server creation, you can't have the credentials again.
However you can do it yourself, if you want.
For the server you can add your SSH key by visiting SSH keys in your Forge Profile. Afterwards you can SSH into the server:
ssh forge@example.com
or
ssh forge@ip
And change the passwords manually for the root user and forge database user.
Funny that Taylor didn't implement this yet.