I need to change my payment method

That’s not asking you to make a purchase; it is saying your card issuer declined your use of the card. See this Apple support article→If your payment method is declined in the App Store or iTunes Store - Apple Support, and additional information below.

A “payment declined” message means that the issuer of the credit or debit card you have on file in your iTunes account rejected a charge or account confirmation. It is not an Apple problem; Apple is just passing through the message from your issuer. There are a number of reasons the charge or verification might be declined:

1. Insufficient balance or account in arrears
2. Account suspended for suspected fraud (not necessarily by you)
3. Error in card number
4. Wrong expiration date
5. Wrong security (CVV) code
6. Address in your iTunes account does not match exactly the billing address for the card
7. The card was not issued in the country of the iTunes store
8. You are connected through a VPN provider that does not terminate in the country of the iTunes store you are connected to.

I doubt that it is asking you to make a purchase. When you set it up it will place a small hold on the card to verify that the card is valid, but that is not a purchase. See→How apps, content, and subscriptions from Apple are billed - Apple Support, specifically the “pending charge” section.

jasveen11 wrote:

I m trying to change my payment details but after I fill in the card details it’s asking me to make a purchase.

Can you post the exact message that you are seeing? If you take a screenshot, please do not include any personal information.