In order to install any premium theme after purchasing it, please do kindly follow the steps below.

1. Download premium theme file from your account/member dashboard (under Active Resources):
https://volthemes.com/member/

2. Extract the file packages. something like:
“vt-blogging-pro-package-2.1-unzipme.zip”.
the package including ‘parent theme’, ‘child theme’, and link to theme documentation.

3. Login to your WordPress Dashboard >> Appearance >> Themes.

4. Click Add New button at the top.

5. At select input type, select the theme file from your computer.
example: "vt-blogging-pro.zip"

6. Once you select the zip theme file, click the upload button

7. It will be installed automatically. Once it is installed click activate it.

8. Done!

From this point, you may need a specific theme documentation guide.
Kindly visit https://volthemes.com/documentation/ & click on theme guide that you had purchased.