Depending on whether you're tracking site-wide conversions or page specific conversions, you'll need to follow one of the following instructions.
Looking to learn more about tracking pixels?
If the purpose of the pixel is to track conversion on a specific page, then it should be place at the top of the page template of that page – not in the theme files. The page template is found under Page > Template.
If the purpose of the pixel is to track site-wide conversion, then it should be placed in the layout.html file of a custom theme. This is found under Theme > Current custom theme> Templates > layout.html. Full disclosure, you'll need to have a custom theme enabled to apply changes on the layout.html file.
From here, you'll want to navigate to the layout.html file. Most tracking pixels should be applied within the <head> tag of your layout. The <head> tag should be visible within the file (see example below).