If its managed by a vCenter:

1. let the ESXi join the Windows AD too and you can log into ESXi with AD credentials and change password
2. With the help of a Host Profile you can set a new ESXi root password as well (needs Enterprise Plus)

or when its a ESXi 6.7

1. Booting with a Linux LiveCD or the gpartedISO and extract bootbank TGZ and mount it. You can that use "passwd" to write into the etc/shadow. Done that a dozen of time in the last years. It took 15min.

Regards,
Joerg