How can I add access to the Sliders for Editor users?

If you open up this file: \wp-content\plugins\nextend-smart-slider2-full\nextend-smart-slider2-full.php
Then around line 47. you can find this code:

add_menu_page('Smart Slider 2','Sliders','manage_options','nextend-smart-slider2','nextend_smart_slider2','dashicons-format-gallery');

Change the manage_options word to edit_pages and your users with Editor roles will have access to the Smart Slider too in your backend:

add_menu_page('Smart Slider 2','Sliders','edit_pages','nextend-smart-slider2','nextend_smart_slider2','dashicons-format-gallery');