Downgrade account

From bronze to silver

Sorry, I'm not sure if I understood your question, you used "Downgrade account" in your title, but then you said "From bronze to silver", that's actually an upgrade. 

Anyhow, if you want to upgrade you may follow this guide: https://www.jotform.com/help/331-How-to-Upgrade-My-Account

If you need to downgrade your account back to free, then you may follow these guides: 

https://www.jotform.com/help/341-How-to-Cancel-a-Subscription-and-Downgrade-Back-to-Free

https://www.jotform.com/help/336-How-Can-I-Cancel-My-PayPal-Subscription 

Thanks.