Obviously this isn't something that you would do a lot, but in case you need to know how, here's how you can reset all user passwords in a Microsoft Office 365 environment:
Help on accessing Office 365 powershell:
http://onlinehelp.microsoft.com/en-us/Office365-enterprises/hh124998.aspx
Once your connected run the following commands:
Import-Module msonline
Connect-MsolService -cred $cred
Get-Command -Module msonline
Get-msoluser |set-msoluserpassword -newpassword "password" -forcechangepassword $false
That's it! :-)