Changing a user's time zone is easy.
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​NOTE: existing events on the user's calendar will keep the timezone assigned when the appointment was created. To update the timezone of existing events you will need to reschedule these events after you change the setting.

Step 1: Go to Settings > Team Members

Step 2: Select "Preferred Time Zone"

Step 3: Save

This setting can only be changed by logging into the Web App at this time.