Users that subscribed to ChatGPT in the Apple App Store can use the Restore purchases feature in the ChatGPT iOS app to restore their subscription on their iOS device.
Under Settings -> Account -> Restore purchases
Note: The Settings screen may look different from this image. In the current iOS app, Restore purchases is still available in the Account section.
Note: Users who subscribed to on the web will need to manage their subscription at chatgpt.com.