If the pixiv account you have linked to Pastela is subscribed to pixiv Premium, a "pixiv Premium" badge will be displayed on the following pages.
▼App: Canvas list
▼App: Account & cloud settings
▼App: Plan settings
▼Pastela Cloud (website): Account settings/Top-right menu
*You must be logged in with your pixiv account to view this.
*If the window is narrow, you can open the menu via theicon in the top-left.
If you would like to subscribe to pixiv Premium with your currently logged-in (linked) account, you can sign up here.
*Please make sure you are using the correct pixiv account when subscribing.
https://www.pixiv.net/premium
If you would like to change the pixiv account linked to Pastela, see:
Can I change my linked pixiv account or add a new one?