How can I change my payment method to PayPal?

I am currently paying by credit card but would like to change to either PayPal or via mail if necessary. Nowhere am I seeing the option to edit my payment method--I only have the ability to change to a different credit card. When I downgrade or upgrade plans, I'm only given a confirmation message; I'm not asked for payment info.

Unfortunately, there is not a way to switch the payment method from Stripe (the current you're using) to PayPal or any other, as you've already noticed you can only switch to a different Credit Card. 

I can see you have only changed your subscriptions, but did not cancelled it so I would suggest you to cancel your current subscription and then upgrade to the desired plan, but after your subscription has been completely downgraded, otherwise it will use your current payment method. 

Once your account is set to free you should be able to see the pop-up with the payment methods available, make sure you're upgrading with non-profit discount since it has been approved for your account. 

Example upgrade pop-up: