If you are in the process of adopting or migrating to Office 365, you will probably be looking to automate as much of the process as possible. When a user initially logs in to Office 365 they will be prompted to set their Regional and Language settings.
The following command can be used to set the time zone and language settings across all of the mailboxes.
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Get-mailbox | Set-MailboxRegionalConfiguration -Language en-gb
-TimeZone "GMT Standard Time"
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By piping the Get-Mailbox string into the command it sets it for all mailboxes, you could of course use the -Identity switch and and point to individual mailboxes/users.