How to process unsuccessful subcription upgrade with Stripe

I am new to Stripe and I can't figure out how to handle the such situation:

A client has an active subscription. It is paid for N days
Before ending this subscription, the clients try to upgrade it to a more advanced
Stripe tries to make a payment but it fails
After several fails stripe cancels the subscription

Instead of this, I want to revert the subscription to the previous one, because there are still N paid days of the previous subscription.

Solution

I'm going to restate your issue here to make sure we are on the same page and provide context for my answer

Problem

A customer attempts to update their subscription, which requires a new payment
The update succeeds (the Subscription is upgraded) but the new payment fails.
The subscription cancels after repeated failed payment attempts

Solution

You can configure how a modified Subscription responds to a payment failure by changing the payment_behavior parameter when making the the Update API request. In your case you want to provide pending_if_incomplete.

This will only change the Subscription if payment succeeds. You can read the overview here and get details about parameter restrictions and the lifecycle of pending updates here