I am already using child theme, but what have I to do?

Have I to use the attached theme instead of the downloaded one?

what custom code have I to use?

If you have not made any customisations in your child theme then you can just delete it and use the above shared child theme.

If you have made customisations in your child theme then please share me your child theme so that i can provide you new child theme merging code from both child themes.

there is a way to hide the text/excerpt in the preview so that I can solve this issue in a simple way?

Could you please share me the page URL from your site where it is displaying so that i can help you to hide it?