Update Card or Account to Be Charged
The customer can do this themselves. If they used Stripe to pay they can go to their account page on your site, open the Subscriptions tab, and click the ‘Update' link to enter their updated credentials. If they are using PayPal, they will need to log in to their PayPal account and update the payment method in the subscription there.
Change PayPal Account
If the user wants to change the PayPal account that is being charged for the membership, you will need to cancel the membership and then have them sign up again. You can either have them sign up after the current term ends with the new payment information (recommended), or you can have them sign up right away and create a coupon that will give them a prorated price.
Switch from PayPal to Stripe or Vice Versa
Sometimes a customer may have used PayPal or Stripe when they originally signed up, or they used Stripe and want to switch to PayPal.
If the customer is using PayPal and just wants to change the payment method, they can do this from within their PayPal account and do not need to switch to Stripe. But sometimes a customer will not want to update their PayPal account (or won't be able to access it anymore) in which case they would need to switch to Stripe.
In these scenarios, you would need to cancel their current membership and have them sign up again. You can have them wait until their current term ends (recommended), or if you want them to sign up again right away you will have to create a coupon for them to use to give them a prorated price.