Patch replace requests returns 204 but does not update next payment amount on sanbox

[busraUcak]

Hi,

I am trying to update next payment amount on sandbox environment. My request details are below

https://api-m.sandbox.paypal.com/v1/billing/subscriptions/BILLING_ID

[{"op":"replace"

,"path":"/plan/billing_cycles/@sequence==1/pricing_scheme/fixed_price"

,"value":{"currency_code":"USD","value":"59.99"}}]

This request returns 204 but when I check on sandbox environment, I can see it is still 5.90.

If it is already updated why it still shows 5.90 in payment details?

And if it cannot updated why it returns 204 and what is the issue?

EDIT: my payment will start 7th of June and I want to update next payment. Currently total billed amount is 0.00$