If I stop paying for upgrade iCloud package what happens to my data

You will lose access to it and it may be deleted or your account terminated. A subscription is a contract, and you agreed to either pay, or cancel. If you do neither, Apple has the right to do any of those things with your data or account.

See -> Legal - iCloud - Apple

”The applicable fee for an upgraded plan will take effect immediately; downgrades to your plan will take effect on the next annual or monthly billing date. YOU ARE RESPONSIBLE FOR THE TIMELY PAYMENT OF ALL FEES AND FOR PROVIDING APPLE WITH VALID CREDIT CARD OR PAYMENT ACCOUNT DETAILS FOR PAYMENT OF ALL FEES. If Apple is unable to successfully charge your credit card or payment account for fees due, Apple reserves the right to revoke or restrict access to your stored content, delete your stored content, or terminate your Account.”