Upgrading your account is easy, and can be managed through the Settings menu.
After logging into PlayBooks, click on the drop-down menu along the top-right of the app, then click on Settings:
Now click on Billing:
Next, you'll see the current plan you are on:
To change this, click on the plan name and then click Edit Subscription. From here you can upgrade your subscription by clicking the drop-down menu, choosing your plan and clicking Update Subscription.
You'll be asked to confirm your payment details after that.
If you get stuck or need more help, just hit the pink live chat icon along the bottom right of you screen.