Installing a new template is easy, click on the ZIP button and upload the ZIP file of the template you downloaded. Do not change the ZIP file name or names of the files inside, doing so will result in your template not working.
Once the ZIP button is green, click the 'Install New Template' button. After it's installed you can use it by changing the current template under Template Settings (section 6).
Every once in a while it's great to have a backup of the content on your website - just in case. Click Here to download a backup of your website.
If you need to restore your website from a backup, choose the backup file (usually called data.txt) and click 'Restore From Backup'.