I already have Shapr Pro, but I'd like to upgrade my subscription

With IOS:

You need to go to the App Store settings and choose the offer you're interested in.

You'll then pay prorated by the number of days with the previous offer.

At the end of your first subscription, Shapr will check the subscription validity:
- If Apple confirms, the dates are automatically changed.
- If Apple doesn't, you'll have to "Restore Purchase" in the app settings. Don't worry, you won't pay twice.