If you tell us which citation style you're using we can help you better.

Add this to the English locale section. That means, add this after line 44.
<term name="retrieved">Download on</term>

This basically redefines retrieved to download on.

Sorry, I forgot to send this link along: https://www.zotero.org/support/dev/citation_styles/style_editing_step-by-step

You can use this: https://editor.citationstyles.org/codeEditor/