Have you set up the instance with SSH keys? If so then it's really easy. Just log in with SSH (using your keys). Then you should be able to reset passwords pretty easy. Webmin using the root account for login, so resetting the root password will fix that:

passwd root

WordPress password reset will require you to re-run the inithook. OTTOMH it be should be something like:

/usr/lib/inithooks/bin/wordpress.py

If you don't have keys, then it will depend on where you have the server running. If you are running this locally (either bare metal or VM) then you have a range of options to reset the root user account password. The 2 that spring to mind are using a Linux "Live CD/USB" to boot the machine and chroot into your OS. THe other is to boot into single user mode on boot.

Sorry I haven't given full details or step-by-step for those possibilities. I don't actually recall the steps OTTOMH but you should be able to find plenty of info via google. Note that under the hood TurnKey is Debian (v14.x = Debian Jessie). Ubuntu is also derived from Debian so most info relevant to Ubuntu servers should also apply.

Hope that helps.