To subscribe to the Premium plan you just need to:
Click on your form.
From the options above, press on the Upgrade button.
Adding unlimited number of fields.
Unlimited form submissions.
Export your data to CSV, TXT, XLS files.
File Upload and Signature Fields.
Pricing:
The monthly subscription costs $4.99. However, if you subscribe annually you'll save 20%, paying $3.99 per month.