You can switch themes in the "Design & Assets -> Theme gallery" section of your stores' Toolbox. Before you change themes, you should always download a backup copy of the theme that your store is currently using. This can be done in the "Design & Assets -> Upload/download theme" section.
Once you have selected your new theme, you should go to the "Design & Assets -> Theme editor" section to set up your new theme, and you will definitely need to consult our documentation on your new theme.
Your new theme may include different menu configurations, different slideshow sizes, and different product image size requirements. You should consult our documentation to find all of this out BEFORE you switch themes.
If your store is already live, it's a good idea to create at least one slideshow image BEFORE you switch themes, so you can quickly upload that once you have switched themes. Please consult our documentation for creating slideshow images.
If you do switch themes, and find that you're unable to get your store working in your new theme, please contact us for help, or upload the backup of your theme to switch back to how your store was originally. This can be done in the "Design & Assets -> Upload/download theme" section.
It's important to download a backup of your theme before switching in case you want to switch back.