Step 1 : Go to your Shopify admin site and click on "Online Store" link from menu.

Once you’ve open the "Themes" section, you will be able to see the "Customize" button in the right side of the screen, just click on it.

 

Step 2 : Now go to the lower left side corner, where you will see an option "Theme actions" click on it, this will display a list of options where you'll find "Edit code" option.

Step 3 : Now go to the "Layout" folder and click on it to display a .liquid files list, now click on "{/} theme.liquid", this will automatically generate a code file in the right side of the screen.

u can add your pixel code, just above  section. Save and refresh your site to see the results.