Go to settings under your name in the bottom left of the screen.
Select Profile.
Select Billing.
Select the Plan you desire and fill in your required information.
Once your upgraded plan has been selected, you will be redirected to Stripe, a secure online payment platform.
Enter your email under Contact Information.
Then enter your payment method and information. Once that has been submitted, click "Subscribe" and you should be good to go!
You will receive a confirmation email from stripe providing you with a confirmation and information regarding your purchase.