Never heard of prestashop but if it has a built in update feature (usually somewhere within the administration menu, maybe under the modules list for updates and such) then you should just do that. Otherwise if it's PHP base you should be able to just download the latest zip file, extract, and upload it to your site overwriting existing files, then when you go to the site it should go through the final update steps. I would recommend that you refer to prestashop's support site though and read any readme files within the zip though for proper upgrading guide.