For some reason this is not enabled by default for the Administrator user.

To enable it:

sidebar at left, Permissions
Admin roles
double-click organization management
Under "Roles:", click +
Choose "Reset Password" from the list

If you get "You don't have access to create, change, or remove the "Reset
Password-Organization Management" management role assignment. You must be
assigned a delegating role assignment to the management role or its parent in
the hierarchy without a scope restriction.":

Run these commands in exchange management shell:

[PS] C:\Windows\system32> Add-pssnapin microsoft*
[PS] C:\Windows\system32> Install-CannedRbacRoles
[PS] C:\Windows\system32> Install-CannedRbacRoleAssignments

then log off ECP and and back on, and retry.