There are two ways to upgrade your subscription.
First, you may encounter a feature that is only available on a higher plan. For example, if you are on the Freelance plan and want to edit Settings such as Merge tags and Special links, you would be prompted to upgrade.
Additionally, you can select My subscription from the account menu in the top-right corner, and click on Change plan
In both cases, you will then be taken to a page where you will see a summary of available plans, with an indication of their main features, the plan that you are currently on, and a button to upgrade (note that the features and pricing information on the following screenshot may be out of date: check your BEE Pro account for accurate information).