How can I change App Store in-app purchase passcode?

Re: "how can i change the passcode i set for in-app purchases?"

If you use "screen-time", there may be privacy restrictions turned on there.

If needed: Learn how to reset your Screen Time passcode

What to do: If you forgot your Screen Time passcode - Apple Support

To change your app-store settings:

See: Manage App Store purchases, settings, and restrictions on iPhone - Apple Support

Eg: says:

Set content restrictions and prevent in-app purchases

After you turn on content and privacy restrictions, do the following:

1. "Go to Settings  > Screen Time > Content & Privacy Restrictions, then turn on Content & Privacy Restrictions.
2. Tap iTunes & App Store Purchases, then choose restrictions for installing or deleting apps and in-app purchases.
3. Tap App Store, Media, Web & Games, then choose age-appropriate restrictions such as the following:

• • Apps: Restrict apps by age ratings.
  • App Clips: Prevent App Clips from opening."