As soon as new version is available, you should see a yellow notification in lower-left corner. Click there and confirm you want to start upgrade, it'll be automated process. You can make backups before upgrading, just to be sure in case of any troubles, you can restore the previous version of your website.
Alternatively, you can go back to website installation page, where you started your website and just repeat installation process, where you selected a template to continue - don’t worry, it will only copy system files, your website subpages, data and template should not be lost.
