edit/add settings to admin options pages of a plugin

Hello,
I installed Post Views Counter and I need to modify/add some options within the admin pages already there.
I am a newbie in terms of code development, so I am asking for some tutorial which drives me through the process of adding new custom options.
Anyone could suggest me something?
Looking at this (e.g) was of any help: https://wpengineer.com/2139/adding-settings-to-an-existing-page-using-the-settings-api/
With the above tutorial I cannot add anything to the admin option pages which come with the plugin.

Thank you.
Bye.