Click on your profile picture on the top left corner of your screen

A drop down box will appear, go ahead and click "Profile & Preferences"

Select "Billing"

Next select "Open Billing Portal"

Click "Open"

Go ahead and select "Update Plan"

Go ahead and select which plan you would like to upgrade to and then select "Continue"

Double check and read over your updates, your plan you have selected, your correct billing/credit card details and when the amount will be due. Once you have done this select "Confirm"