Login to your Shopify account, go to Online Store and click Themes in the menu on the left. Then click the Customize theme button.
Click Theme options in the top menu and choose the Edit HTML / CSS option from the drop-down menu.
You will see a list of files and folders. The file you need to edit is called theme.liquid and is located in the Layoutfolder. Scroll to the very end of this file and paste the code you have copied from Feedspot widget page right before the closing </body> tag. Click Save to save the changes.