There are two ways of changing the plan you're on. One is by clicking on the Upgrade button at the top of the side menu.
And the other is by going to Your Account at the bottom of the side menu where your profile picture is, and then going to your Settings page.
Then, scroll down and you will find a Change plan button. Once you're on the Pricing page, choose the plan that best fits your needs, and follow the steps.
For active subscriptions purchased with a card, the system will automatically detect your current payment and you'll only be charged the difference between both plans. That is, you'll only pay the difference between your new plan and the one you're currently on.
Keep in mind that the checkout page will display the full amount of the plan you're purchasing, but the amount charged will include the discount.
For PayPal payments, it's unfortunately not possible to automatically discount the amount of your current plan. In this case, make sure you contact our support team through the platforms' Chat to get help.