How do I update my MailerLite JavaScript on my website?

[ClaudiaKaz]

Hi all, 

I use MailerLite email platform and have a form embedded on my Shopify website for signups. MailerLite has migrated over to a new version, so I need to update the JavaScript accordingly. Coding is not my thing. Whilst I think I've found where I need to put it, I don't want to break anything. Unfortunately, I can't seem to find my original JavaScript either, to just replace it. 

Can anyone who knows, please give me the simple step-by-step instructions to be able to add it where I need to? I know it's actually very straightforward... if you know!

Thanks in advance!