Go to Settings of your device -> iTunes & App Store -> Apple ID -> Subscriptions -> Wallpapers Premium and choose new subscription plan.
You can turn off auto-renewal at any time from your iTunes account settings, but refunds won’t be provided for any unused portion of the subscription period.
When free trial ends it automatically converts to a paid weekly subscription, unless you turned off auto-renewal at least 24 hours before the end of the current trial period.
You can, also change your subscription plan in your iTunes account settings.
Subscription automatically renews unless auto-renew is turned off at least 24-hours before the end of the current period.