If you want to switch payment method from credit card to PayPal or vice versa, first of all, you will need to cancel your current subscription from here.
Then, please wait for the billing cycle to end. You can check the status from your Billing Info section.
Once you're back to a Free plan, you'll have a two-week window to upgrade back to whichever plan you wish without having to lose any of your content. You'll be able to select the new payment method while upgrading again from here.