I have an App Store subscription with an introductory price with a duration of 3 days. I can see this by checking my SKProduct's introductoryPrice property, which has a subscription period of 3 days:
let product: SKProduct
...
print(product.introductoryPrice.subscriptionPeriod)
I can also check this in App Store Connect:
It says 'free for the first 3 days', but there is no way to edit that. I can only click on 'no end date' and set an end date, which is not what I want. Is that editable and if yes how?
According to the official docs on Set an introductory offer for an auto-renewable subscription, only the end date can be edited once it's available to customers.
Once you create an introductory price or free trial, only the End Date of the in effect introductory offer can be edited once it's available to customers. For future scheduled introductory offers, the start and end date can be edited. To edit the duration or price, you must create a new introductory offer.