lizizcon on "Display only excerpts of posts in home page"

hi all,
after I added categories to my menu, the posts in home page are shown fully,it was all good before I made the change, but now the posts are displayed fully ..I've searched all related cases but no luck.
how can I display only excerpts of my posts on home page!!
please help,what should I do !!
thanks in advance.

(1) install the plugin "show current template" to find out what template is being used to show the content

(2) create a child theme -- https://codex.wordpress.org/Child_Themes

(3) copy the file you identified from (1) into the child theme and modify it so instead of using the_content(), it uses the_excerpt() https://developer.wordpress.org/reference/functions/the_excerpt/

I did the two first steeps but i'm stuck with steep 3 (I'm not a man of the field), and I don't see any 'the_content()' or 'the_excerpt()' in the file I'm trying to edit (index.php)..so I don't know what to edit.
here is a link to the code:
https://drive.google.com/file/d/0Byoo27L1UxRkODdwRlJzYUNYVkE/view
what should I do !!
thank you.

the code is in one of those template parts, post-homepage.php or post.php

the code is not in 'post.php' file, and 'post-homepage.php' file does not exist (it is only to be created apparently).
for info:everything was working well before I added categories to menu. but since I put categories in place of menu items, my Home menu link disappeared (I only created a link referring it to home page) and post started displaying full on home page.