You can change the name of the function along with the needed modifications and copy it to child theme’s function file, then in original file you need to search for add_filter or add_action and in child theme you can unhook original functions using remove_filter or remove_action and the you can use add_filter or add_action with your new function.
In short you can simple find the add_action or add_filter for these modified functions, copy them and just rename from add to remove, then again you can copy the add_action or add_filter from the original theme and this time you can replace the name of the function.
Update:
For this specific theme, you don’t need add_filter or remove_filter because the function flexia_post_large_title_author_avatar_markup is directly called at line 174, and maybe it’s not used at any other place so it’s not hooked to any action, and same for the second function