First of all, you must notice that you can simply hit the "Upgrade Account" button located on the header bar, or you can click on your profile picture and a menu will be opened. If you look closer in your menu, you will see that there is again your "Upgrade Account" button.
After you hit the "Upgrade Account" button, you will see the three plans you can upgrade to:
Click the "PURCHASE VIA PAYPAL" button and then you will have to fill a form that looks like this (if you don't have a PayPal account:
And like this if you have a PayPal account:
You will log in and then you will pay the fee. That's all. Your account is upgraded now!
Updated about 5 years ago