Currently Neartail does not have a test mode for Stripe payments. Once you have enabled card payments in your form, you will have to submit a response and make a payment if you would like to test it.

I want to chnage the payment endpoint, so if I change thatbthen I need to update the new endpoint keys in Neartail. how can I do that?

When you click on the configure option for card payments, you will be able to create a Neartail Stripe connected account. This stripe account will be linked with your Neartail account and it cannot be changed. All the forms you create using your account will use the same Stripe account for payments.