First, you will need to download the latest version of your theme:
If you bought the theme on ThemeForest, go to the ThemeForest downloads and download the installable zip file (installable WordPress files only).
Unzip that file, you will find inside the folder called the same as the theme (it. newscore, newsfront, energia…), where the theme files are located (the root folder of the theme is the one which has the style.css file in it).
Overwrite all the files in the current version of the theme (wp-content/themes/name-or-your-theme/) with the new files.
If you are doing this on your local server, just copy/paste the entire folder and overwrite all the files.
If you are updating remote (live website) server, you can do this using FTP, SFTP or SSH protocols.
Make sure you make a backup of files and database in advance, just in case!
I strongly encourage to test the things locally first or on the test/staging server, because some options might be affected with the updates. However, you cannot loose the content (pages, posts, services, galleries, projects …) during the update, because all the content resides in the MySQL database and is not affected by the update.
If your theme uses the bundled plugins, make sure you update those too!