The timezone settings allow you to set a default timezone used for every date/time display generated on the site. To access the timezone settings panel click the "Settings" > "System Settings" link from the top menu. You will be shown a list with all the available settings for your installation. Find the "Timezone Settings" section and click the "View/Edit Settings" button or double click the row. To change the timezone in use, select the new timezone from the "Timezone" drop down menu.
After that click on the "Update Settings!" button. The settings will be saved but will not be applied to the site. To apply the new settings you will have to rebuild the configuration file. To do that find the green "Apply Settings" button above the form and click it. The new timezone settings will be in use as of this moment.