You can upgrade your PubHTML5 account as you wish.
1)If you are a free user, you can upgrade your account by ordering the Pro, Platinum or Enterprise package in our website: http://pubhtml5.com/pricing.php.
2) If you are a Pro user and you want to upgrade to Platinum package, just directly upgrade your account on price page: http://pubhtml5.com/pricing.php; When you want to upgrade to Enterprise package, you need to cancel the subscription of Pro in your PayPal account first (https://www.paypal.com/selfhelp/article/FAQ2058/1) and then purchase the Enterprise package.
3) If you are a Platinum user and want to upgrade your account as well, please follow the same steps as the Pro user.