To purchase a plan, log in to your account and go to the page with the list of your projects. Go to the account settings:
Go to the Pricing and plans section and click on the Upgrade button:
Choose the plan that suits you best
Next, choose the plan period.
At the next step, you will be redirected to the PayPal website to make the payment:
Log in to your PayPal account and follow instructions on the website.