If you are installing a template in a working store, make a backup before.
1.Log in to your store's admin panel, then go to Preferences-> Themes and click on the "Add new Theme" icon.
In the "ZIP file" field, select the installation file of your template and click "Save".
Add the installation file to the theme folder using FTP, then select the archive in the "IMPORT FROM FTP" field and click save.
3.Click "Use this theme".
4. On the next page, click the "Save" button.
5. On the next page, click the "Finish" button.
Go to Advanced Parameters->Performance select "Recompile templates if the files have been updated" and save.
Next click "Clear cache".
if the comments module is missing after installation:
Go to Modules and Services->Modules and Services and find the module "Product Comments" and install it.
If you can't find module you can download it here:
Clear again PrestaShop cache after instalation.
If after installing the theme the product photo are different size than in the demo, go to Preferences->Images and click "Regenerate thumbnails".