You will be given the option to downgrade to Shift Basic once your trial is over. Head into the Pro tab and select downgrade.
If this does not work, please try resetting your application data. To do this, go into Settings, General, and then "Reset app data" which can be found at the bottom. You will need to re-add your Shift accounts, and please ensure you are selecting the same Primary Shift Account as the one you had before.
If you still cannot downgrade after following these steps, please contact us at support.
If you are looking to downgrade from Shift Pro to Basic, please turn off auto-renew in the Pro tab of settings. Please note that your Shift Pro subscription will still be valid until the scheduled end date displayed on the Pro tab.