Sorry, our current version doesn't allow the users to change the payment method after they initiated to payment. I've added this to our TODO lists and I assure we will have this in future.
But, you can set "PayPal" as the default payment gateway as another solution. Kindly follow the steps below to achieve this,
1. Open {root}/administrator/components/com_yendifevents/libraries/fields.php
2. Locate the function "ListPaymentMethods"
3. Modify the "Order BY" statement from the query as " ORDER BY ordering ASC"
4. Save the changes.
5. Login to your Joomla back-end
6. Go to Extensions => Plugins
7. Filter by "type = yendifevents"
8. Move the row related to "PayPal" to the top
That's it. Hope, this solved your issue !!!