Login and go to your My Profile page, by clicking on your name at the top-right of the page.
In the Membership details section, click on the button that reads "Renew to ... "
If there is no button visible, it is not time for you to renew yet.
Verify your information is up to date, especially your Emergency contact information, then click on "Update and next".
You will then be given a choice to be invoiced, or pay online.
If you chose to pay online, you will be sent to a page like the one pictured.
If you have to come back later to make payment, you can go into your profile and under the Invoices and payments tab, you can view your invoice and continue.