Hi again, Huiwen.
They can enter it at your-site-com/events/community/payment-options
The data is stored in the wp_usermeta database table under the tribe_events_community_tickets meta_key, but it’s in a serialized array so it really shouldn’t be modified (input, edited, or deleted) via the database.
You could use a plugin like https://wordpress.org/plugins/user-switching/ (third-party, not guaranteed or supported by us) if you already know which PayPal email address belongs to each of your users.
Please try this snippet (or use it as a starting point).
Please let me know all this goes for you.