First of all, you need to download the theme installable file from your account that purchased item, go to your Dashboard > Downloads Tab
If you are updating your website theme from an older version then please make sure taking Backup of the complete website Files and the Database for any future reference.
Install theme via WordPress
Step 1: Navigate to Appearance > Themes
Step 2: Click Add New, then click Upload Theme > Choose File
Step 3: Navigate to the .zip file on your computer, then click Install Now
Step 4: When the installation completes, click Activate. You will be redirected to the Themes page with Rustic activated.
Install theme via FTP
Step 1: Unzip zip file package – the zip package is inside the Rusticfiles folder you have downloaded from EnvyTheme Rustic.
Step 2: Connect to your FTP server and navigate to the folder where your WordPress is installed.
Step 3: Go to wp-content/themes/ and upload theme folder (the path will look like this: wp-content/themes/runhill)
Step 4: Open WordPress admin panel (navigate through the web-browser to the folder where you have installed the WordPress e.g www.yourdomain.com/wp-admin/) and go Appearance > Themes on admin sidebar. Activate Rustic