I should say that I know nothing about HTML.
It depends on what the file is eg a HTML template, WordPress theme etc
If it’s HTML then you need to open the files in an editor and make necessary amends.
If it’s WordPress then you need to install it on your domain/hosting then can edit it from the WP admin.
There will be documentation with the file but that will only cover the basics and not explain how to change everything or work with the file in detail - there is an assumed level of knowledge when it comes to buying templates and themes.
You can always look on www.studio.envato.com to hire a freelancer to help you (it would be wise to know exactly what you want first otherwise this could get expensive).
thanks. I guess I didnt know that.