Resetting a user's password is incredibly easy. Find the Reset password button at the top of the single user options dialog, just below their name and to the right of their initials/image. In the bulk user menu, it's near the bottom of the list. In either case, once clicked, the options will look exactly as they were when you created a new account:
To reset a user's password to a random temporary password in PowerShell, use the following:
Set-MsolUserPassword -UserPrincipalName "jdeer@liquidhgdev.onmicrosoft.com" -ForceChangePassword
Note that you'll have to somehow tell ...