Go to Shopify admin > Online Store > Themes > click the "Actions" button on your published theme > Edit code.
In the page that open, click on the theme.liquid that will appear under Layout folder.
Press Ctrl + F to open the search box inside this theme.liquid > type "qikify".
There will be 1 line containing the word "qikify" > remove that line, it will be: {% render 'qikify-apps' %}
Click "Save".
Stay still on the "Edit code" page from step 2, type "qikify" in the search box on the left panel.
There are 5 files containing "qikify": qikify-apps.liquid, qikify-boosterkit.liquid, qikify-boosterkit-data.liquid, qikify-boosterkit-offer.liquid, and qikify-boosterkit-data.js
Click on the "Trash" button on each file to delete them.
Done. qikify Upsell Popup Cross Sell is now removed completely on your store.