When your theme was installed, we installed some galleries onto your account. You can read more about galleries here.
If you want to change the images in a gallery then you'll need to read the instructions in this article.
If you want to change the actual gallery that's used on your website, for example you may have created a new gallery of images to display for a special offer or over a certain period (like Christmas). If you do then you can follow the instructions below.
Assigning a new gallery
Go to the theme settings page (which can be accessed by clicking 'themes' from the left menu and then 'theme settings').
Scroll to the galleries section.
Most of the time your theme will only use one gallery (but some themes have more).
Select the new gallery from the drop down list and click 'save changes' at the bottom of the page.
When the changes have been saved you should check your theme to make sure the change has been saved correctly and the gallery is displaying how you want it.