Upon logging into your Free AWeber account for the first time, you'll have the opportunity to upgrade to AWeber's Pro plan. On this page, you'll find a comparison between what is offered on the Free and Pro plans, as well as pricing information.
If, after accessing and using your Free account, you decide to upgrade to the Pro plan, you can accomplish this from within your account by visiting the Upgrade page. You can also navigate to the My Account section, found in the dropdown menu when hovering over your company name and email address in the top right corner of your page.
Then, select Billing. There, you will find the same page that was initially displayed upon logging into the account, through which you can upgrade to the Pro plan.
Don't have an AWeber account yet? Sign up for free here.