Due to the way the App Store works, only you can cancel or change your trial. You can access your subscription options as follows:
Open Pillow on your iPhone.
Tap on the "Settings" icon in the top right corner.
Scroll down to find "Pillow Premium".
Tap on "Manage".
Alternatively:
Open the App Store app on your iPhone or iPad.
Tap on the profile button in the top right corner.
Select "Subscriptions".
A screen should appear allowing you to modify your subscription or trial.